first thing, unless you are doing heavy duty drag racing where a modified powerglide could be a good choice, you need a 4 speed trans, with the overdrive for the highway.
Then, investigate which one: th200r4 can be modified for fairly high power (but has to be modified); th700r4 is a good overall trans,even stock for moderate power; 4L80E is the 4 speed version of the old th400, heaviest duty stock, but very heavy (probably not good behind a mild sbc).
So now, just talk to shops, read forums, check ratios... to decide between the 2, th200 or th700.
Easy!