I agree with Tom H that generally speaking the Verizon iPhone is the better choice (unlimited data is a very cool option to have, coverage is better, and it is economically more attractive).
Except if you travel outside the US with any frequency and expect the iPhone to work as a phone. Then you are screwed with Verizon. The Verizon version of the iPhone does not connect to cell networks outside the US - there you have to find a wifi spot and use Skype.
Then again, AT&T's international roaming is very expensive, so you may as well not use it.
OBG