I cant speak for the DFI, you'll haveta get info from another user, but the FAST:
Can have bank to bank or sequential injection
If bank to bank you get cooling fan control, fuel pump control, and knock control. In sequential you have fuel pump control, individual cylinder correction, NOS control (wet or dry).A DOS based or Windows based interface. Help is pretty good. I actually wrote a nicer more complete help file, but I havent had Lance's approval yet so its not official.
Do you need bank to bank or sequential? Hmm..tough call. Most of the people I know run a Bank system. I think it depends on how hard you're gonna be pushing the injectors. I read somewhere, maybe Accels website, in a bank system, of you're close to 100% DC then the unit gets real hot and can shut down as the drivers overheat (6 or 8 injectors being opened staticly) but the sequentials in that case run significantly cooler as it is only opening one injector at a time. Add to that it must inject at least a little more accurately (less wasted fuel sitting behind a closed valve) but its like $500 more. My car has a B2B with WBO2 in it and I have it running pretty well. I dont want to spend a whole bunch of time tuning it right now because I have another motor going together (if it ever gets done) and tuning will start all over at that time.
The FASTs editing interface I think is excellent, although I wish there were more hotkeys, or a way to navigate the windows version with the keyboard. I think its a pain in the a$$ to reach over while driving and navigate the file menu's with the mouse/touchpad. It would be nicer to have the features of the windows version, with the navigation of the DOS version in one package.
I dont think you can go wrong with either unit. I was working on a TCC control, but never finished it. Then I heard Caspers was working on the same thing..so why bother. Basically, under 25% load in 3rd or 4th gears my box would count to 5 and lock the converter. Once you downshifted to 2nd or went above ~25% throttle (TPS ~1.7v), it would unlock. Of course the Gen7 has this built in, and more refined than mine