They are completely different animals, other that being automatics and three speeds. How seriuo are you going to build your motor would be my first question. A TH350 is a damn fine tranny and you can build them to take some seriuos ponies with very little work TH400 is a great tranny too, heavier duty than a TH350 basically. You generally see 400's in trucks and mated to big blocks (although I have seen my share bolted to a small block) and 350's are usually in cars and bolted to small blocks. The 400 will rob you of more power than the 350, which will rob you of more power than a stick. I know this is very general info, but I need more info on your car, engine, etc... to say much more. What are you buildin'?, what's it going to do i.e.: daily driver, drag racer, street and strip, road racer. Give me a little more info and I could probably be of more help.