Let me know if I'm missing anything, something needs clarification, or I'm just telling lies.
One of the takeaways of this document is that finding a sewing machine dealer who is willing to work with small-time sewers is very important. Maybe more important than knowing what machine you want first because a good dealer will help you choose the best machine for your application. If you're lucky enough to have a brick & mortar shop nearby, that's great. If not, the internet has opened up a world of possibilities. Here's a very short list of dealers who I like or who come recommended, to give you a start:
USA
Midwest: http://tolindsewmach.com, http://www.nickosew.com
North: http://www.techsew.com
East: http://keysew.com
West: http://www.leathermachineco.com
South: http://www.campbell-randall.com
Beyond North America
Oz: http://www.annebonnyslocker.com.au/ablesew/index.htm
UK: https://www.college-sewing.co.uk
Before we start with the fun stuff, here are some things to keep in mind while you search for the perfect machine:
1. If this is your first industrial machine, buy it from a dealer who will spend the time answering your questions and troubleshooting your problems, whether it’s at the shop or over the phone. It’s worth the extra money to buy a machine from someone who will support it.
2. Clones: That galaxy far, far away is here. One of the more confusing things about shopping for industrial machines is that so many of them look alike. Because of China and Taiwan and the fact that anyone who orders enough units can have their name put on a machine, you may have five or more different brands to choose from in addition to the name-brand model. (In alphabetical order, the most popular name-brands for these machines in the US market: Adler, Brother, Consew, Juki, Mitsubishi, Pfaff, Seiko, Singer). Sometimes the machines are exactly the same, sometimes they’ve been slightly altered to get around patent laws -- the casting might be different, screws are different sizes, etc. Clones are a good way to save a couple hundred bucks if you buy from a reputable dealer who will support it. Sewing machines rarely come from the factory fully tuned. You want a professional sewing mechanic to do that for you.
3. To maintain some sense of order, I’ve broken these machines into three different classes: Upholstery, Canvas, and Extra Heavy Duty. I’m defining an upholstery machine as one that has compound feed and can handle material up to 3/8”. My definition of a canvas machine is one that lacks the compound feed, but can handle similar material (or slightly lighter). And the extra heavy duty machine is for dense material over 3/8”. Some XHD machines can be adjusted to sew lighter material, but if you plan on doing a lot of sewing, you’ll want a upholstery or canvas machine, too. Sadly, in the industrial world, there isn't a machine that does it all. Generally, industrial machines do one thing very well.
4. Many of the listed machines come in different flavors or subclasses: with optional reverse, bigger bobbin, thread cutter, safety clutch, double needle, different feed dogs, feet, etc.
5. Motors: Unlike home machines, most industrial sewing motors are not part of the sewing machine (aka the head). They're mounted under the table and drive the head with a v-belt. There are two basic kinds of motors: clutch and servo. Clutch motors are always running and can go from stop to super-fast in no time. Many new sewers find them intimidating, but with experience, they can be tamed. For great slow-speed control right outta the box, a servo motor is the way to go. There are brushless versions and ones that have extra bells and whistles, but I don't know much about them.
6. Accessories: Generally what comes with a new machine is a few bobbins, one set of feet (the ones on the machine), a bit of oil, and a 10-pack of #22 needles. You might want different kinds of feet, different sizes of needles, edge guides, folders, binders, another light, more oil, etc. Don't forget to ask.
7. Again, there are many, many more machines out there that will do the same task. The goal of this document is to give you an idea about what’s out there for when you talk to your sewing machine dealer.
8. Watch this video from Springfield Leather Company.
A word on used machines:
For first-time buyers, a new machine with a warranty is a safe bet (and probably your best), but there's no denying that old machines are cool, and sometimes a great deal. These machines were built for factory work, so they'll last a long time with proper care. This is especially true of some older Singers, which where designed to be rebuilt. That said, if a machine has seen heavy factory use (as apposed to someone's part-time upholstery business), it may be used up. This usually isn't an issue if you're buying a used machine from a dealer (though "rebuilt" seems to have a broad definition), but it is if you're getting one from eBay or Craigslist. That's why it's best to test used machines in person. I know that's not always possible, so at least try to verify that the machine sews and that the seller knows how to properly package the machine if it's being shipped. For all their heft, these machines have a number of delicate parts, and many a machine has been destroyed in the post. Here's how to build a shipping crate.
Many older machines do not have a reverse. This may not be an issue for you. Stitches can be locked by either turning the work around 180 degrees, or by starting a run and then lifting the foot, moving the material back to the start and sewing over the first stitches.
Canvas Machines
The Singer 31-15 is a drop-feed machine with oscillating hook. Back in the day, it was used to sew flight suits. It’s good for canvas and light leather work. It's no longer in production, of course, but they're easy to find used.
The Sailrite Ultrafeed LSZ-1 is not an industrial machine, but it sews canvas and light leather well. It’s a copy of the Thompson Mini Walker (a beefed-up domestic machine with a walking foot) plus Sailrite's patented zig-zag movement. (The red version is straight-stitch only.) These are easy to maintain and portable and good gateway machines to full-size industrials. They are also expensive, have a relatively tiny class 15 bobbin, and don't make as nice a stitch as compound-feed machines (in my experience). Unless you live on a boat, or really need the walking zigzag, go with an industrial machine.
The 8700 is Juki’s basic drop-feed, straight-stitch machine. It comes in an H version for heavier applications like denim, canvas, and upholstery-weight leather. The regular version makes a great tailoring machine and can do light canvas-work (though, I've seen people make messenger bags with these ... probably not great for the machine in the long run).
Needle-feed machines are able to sew faster than needle-feed, walking-foot machines and have a lighter touch, which makes them good for sewing things like waxed canvas, where foot marks are undesirable. Because new NF machines are pricier than new upholstery machines, you see them more in factories than in small two-machine shops where maximum feed for the money is desired.See also the Singer 211G157.
Upholstery machines
Compound feed, unison feed, triple feed: these all refer to a machines with a walking foot and needle feed, which makes them great for sewing over thick seams, binding and webbing. We’ll look at three different bed shapes: flat, cylinder, and post. These machines generally have an upper limit of 3/8” with #138 thread, though some can be adjusted for heavier work and #207 thread. Keep in mind, too much sewing in the outer limits can shorten the life of your machine.
Flat bed
There are two basic kinds of flat-bed machine: ones with a vertical-axis hook and ones with a horizontal-axis hook. Horizontal-axis machines are generally a bit cheaper and, by some reports, less fussier than their vertical brothers. The bobbins on vertical machines are easier to access (right on top) and can come with the larger U size. Another convenient feature is that the bobbin cover plate can be outfitted with various attachments -- get more plates for more attachments. Bonus.
Horizontal-axis machines
The Consew 206RB/Seiko STH-8BLD-3 and the Juki 1541 are two of the most popular upholstery machines out there and great first machines. They have similar stitch specs. The 1541 is a more modern machine and is priced accordingly (pictured with optional stitch-controlling computer wizardry).
[Note on Consew & Seiko: At one time Consew was the US wing of the Seiko brand. Over the years they've split and rejoined. Currently, Consew's 206 is made in China, and Seiko's version is made in Japan.]
The Juki 241 is the out-of-production precursor to the 1541. Easy to find used.
The Tacsew T111-155 is a popular entry-level upholstery machine. I have one. It does the job fine. You'll get a little more foot lift with a 206, though, for just a little more money.
Don't be confused by Tacony's numbering. The only thing this has in common with the Singer 111w155 is the feet. Same as the Sailrite 111. Looks very similar to Consew's new 1206 (but the Consew has a built-in bobbin winder).
Vertical-axis
You might say the Singer 111w155 is the grandfather of the modern upholstery machine. By all accounts, it's still going strong in shops all around the world. There are a number of machines in the 111w class, but the 155 subclass is the one you see the most. The rarely-seen 156 is the last in the 111w line, and comes with reverse.
The 225 and 226 are Consew's take on the 111w155 and -156. Different guts, but same general machine.
And Juki's take on the 111w class, the 562 and 563. Another old favorite in the upholstery world. Replaced by the Juki 1508. Take note: the 562 has a smaller G bobbin. Also, some 563 and 562 subclasses do not have reverse.
The 255 is Consew's big-bobbin (U) replacement of the 226.
Some of the Juki 1500 series are mega fancy, with square feed, and magic tensioning. The 1508 even comes in an extra heavy-weight version with a longer needle system.
The Pfaff 1245 is another favorite in the upholstery trade, as are its predecessors, the 545 and 145. Make sure you look at the price of parts in your area before you buy a Pfaff -- they can be $$$.
Cylinder-bed
There are three basic kinds of cylinder-bed machines: regular, narrow-arm, and patchers (aka shoe patcher, aka universal feed arm). Patchers are really in a class of their own, but here we are. Cylinder-bed machines come in handy when you need to sew inside of things -- like bags and tubes. You can also get (or make) a table for your cylinder-bed and turn it into a flat-bed. These machines are generally more expensive than flat-beds, but make up for it in versatility.
"Regular"
Here you have your standard Singer-inspired Seiko/Consew here, based on the Singer 153w class.
The Juki 341 is no longer in production, but there are many used ones and new clones out there. Replaced by, you guessed it, the 1341.
You got extra money lying around? Juki made a square-feed cylinder machine.Narrow-arm
For when you need to sew small 3D items. Cylinder-bed machines are commonly used as binding machines. Some narrow-arm machines come with a synchronized binder feature, where the attachment plate moves in synchronicity with the feed dogs. Note: some feed dogs on narrow-arm machines only go forwards and backwards; they do not go up and down. I've had difficulty feeding some things on my 335 that would've been fine on a machine with elliptical feed dogs.
The 335 seems to be the most popular in this class, with many clones.
You can see the older, curvier 335 on the top, and the newer, boxy 335 on the bottom.
I haven't included many Dürkopp Adler machines in this list. They're pretty expensive in the US and not as prevalent as the Sino-Japanese machines, or even the Pfaffs. Though, sewing machines are fairly regional. They might be common in your area. See also the newer, fancier 669. 
The Seiko LSC-8B and Consew 277 are very similar to the 335, but a little less expensive, have simpler internals, and, by some accounts, are a bit stouter.
Patchers
Patchers have been around forever -- and they look it. There’s something primeval about them: long and skinny with a square base, trussed needle-bar lever, and hand wheel. They have a universal top feed (no dogs) that allows you to stitch in any direction with the winged lever. Mostly used for repairs or alterations, you occasionally see them in light manufacturing. Great for mending the toes of shoes, repairing zippers and, yes, sewing patches on leather jackets. They come in a variety of arm and bobbin sizes. The most ubiquitous is the Singer 29k class. The smaller 29k varieties are easy to find used, but frequently suffer from worn feed cams, which limit their stitches per inch. New clones are plentiful, though. Note: these can be hand cranked, treadle powered, or motor driven.
If you want the best and biggest, go with a Claes 30.
Similar to the Claes but out of production is the Adler 30-70.
Post-bed
These aren’t as common as cylinder-bed machines, but for sewing certain shapes, they're pretty handy. The Consew 289RB here is a walking-foot, needle-feed machine. You see a lot of post-bed machines set up with a roller foot for leatherwork. This ensures constant foot pressure for straight tracking on the post-bed's precipitous sewing surface.
The post height starts at around 7 inches and goes up from there.
Extra Heavy Duty
For sewing heavy synthetics and leather you need a heavy machine. There are many old harness machines out there that’ll pound some leather, but here are the most popular ones that are still in production. We’ll talk about two varieties: triple-feed and needle & awl.
Triple-feed
If you need to sew heavy canvas or webbing, the Juki 441 or Adler 205-370 is your machine. With its compound feed and oscillating barrel shuttle hook it can sew through most anything up to 3/4–7/8”. Without swapping out springs and adjusting the bobbin tension a ton, their lower range is around 1/4" with #92 thread and a #20 needle (#22 is the smallest leather-point needle in this system). These are not great for leatherwork right out of the box because of their 
toothy feet and sharp feed dogs. They usually need a speed reducer, too. Plus they’re expensive. This is where the clones excel. They’re far cheaper than the originals and they’ve been modified for leatherwork.The 441-type is the most popular clone. Artisan, Cobra, Cowboy, Nick-O, and Techsew all offer leather-ready 441 clones. They’re all pretty much the same machine. The difference
is in the quality of support and number of accessories you get. Proximity and paint job can also be deciding factors.These come in flat-bed varieties, but most folks just get a table attachment for their cylinder-arms.
It's reported that the Highlead 441 clone has a heavier casting than other clones, but the same internals.
Needle & Awl
If you want the tightest stitches that look the same on the top and bottom, you are looking at hand stitching or a needle & awl machine. An awl pierces the leather, and then a barbed hook comes through and grabs the thread. Depending on the machine, the material is fed by the awl or the needle.
Note: the learning curve for these machines is steep. These are proper factory machines. For factories with full-time mechanics.
Need to sew up to 3/4” leather all day long? The Campbell lockstitch machine might be for you. Need to sew up to 1 1/8”? There’s a high-lift version.
Need to sew up to 3/4” leather all day and do it real fast? Take a look at the Union Lockstitch.
These are used to make Wilson footballs.
Bonus: off-the-grid machines
The Boss is lever driven and has been around for a while.
