I am no expert, but I have owned, used, and dis-assembled several of each of these varieties.
The 83-key XT keyboard has an odd and uncomfortable layout, and is not compatible with modern motherboards unless you use Soarer's converter or something like a Hagstrom unit. Many people believe that it has the best feel of all the buckling spring keyboards. It is very heavy and has a metal back for the case.
The 84-key AT keyboard has a more conventional layout, and will work on a modern computer (if you buy the proper adapter plugs). The entire shell is plastic, and the front edge, at the spacebar, especially, is very hard to pry apart without damage.
The battleship Model F 122-key terminal keyboard is huge and heavy, and also requires Soarer's Teensy. It weighs a ton and has the black metal back.
All these can be taken apart and the barrels and springs moved around to approximate ANSI or ISO layouts, if you prefer (although there is not much to do with the XT in its original case). This is not easy to do because there are multiple curved plates, plus a multitude of individual barrels and springs, that must be held together tightly and slid sideways in a force fit. It is not as bad on the small ones, but the big one is a nightmare, hence my bolt-mod wiki.
All of these topics are discussed at length in several wikis here, although they can be difficult to find.
http://en.wikipedia.org/wiki/IBM_PC_keyboardhttp://en.wikipedia.org/wiki/IBM_Model_M
