700 if you drive it on the street a lot, or just want the overdrie, 400 turbo if you're making real power
The 700 can be built to take some good power though, many don't give it the credit it deserves. With the right convertor, gear and tire combo you can get rid of the 1-2 gap problem. I won't be getting rid of a 700 unless I'm making gobs of power.
350 turbo is lighter, but you'll lose overdrive. If I'm going through the trouble of getting a new crossmember, convertor, shifter and driveshaft I'm making it worth my while and going with a trans that will hold up to whatever i throw at it.
400 turbo, strongest of the 3. You can build a whole trans and not use a single original part. It's a little heavier than a 350 but still less than a 700 and the added strength makes it all worth while.