I just went through this exercise and learned more about these than I ever wanted to.
Bottom line is there are basically three manufacturers.
MTD - Cub Cadet/Troy Bilt/White/Yard Machines and one or two more
Husqvarna - Husqvarna/Poulan Pro/Ariens and perhaps one or two others
John Deere
Until this year most of the Sears machines were built by Husqvarna but they are now having MTD build some of them but not all.
Unless you spend big $$ almost all of them use a Briggs & Stratton engine (some have a Kohler) and either a Tuff Torq K46 trans-axle or a Hydro Gear trans-axle. This includes all the 100 series JD's as far as I can tell.
Some can be purchased with a commercial series B&S engine, the "Vanguard" series.
Having owned both an MTD and a Husqvarna I would not buy another MTD, they Husky is just better built IMO. From what I've been able to find the Tuff Torq trans in the 100 series JD's are actually spec'd slightly less than a standard unit from them to save a few $$ and keep the price about the same as the others.
If I had the $$ I would have bought a 500 series JD but that was way out of my price range. I ended up with a Husqvarna
YTH23V42LS, a 23HP, 42" cut but with the commercial engine, a locking rear diff and a fabricated (vs stamped) deck. Picked it up last Friday and so far it's an animal.
mike