I will probably buy the AT&T 32GB. Mainly because I have 3 iPhones on my family plan and am hoping AT&T comes out with shared data so I can connect my iPad to my plan without paying a separate monthly fee. Plus, the resale value will probably be a little better.
I thought the 3 works on ATT and VZW. Since its not subsidized, you should be able to choose your carrier independantly. I haven't read anywhere that its locked.