We slice and dice the plans, phones and fine print at each major U.S. carrier to come up with 23 reasons you might choose one carrier over another.
There is a rumor as of July that Apple is preparing an iPhone for T-Mobile. This shouldn't be too hard, since T-Mobile is a GSM network like AT&T and will support talk and surf at the same time. If Apple does indeed add T-Mobile, it will probably save the company should the AT&T purchase be scuttled.
This decision comes down to use. The single biggest differentiator between AT&T's GSM network and Verizon's CDMA network is that an AT&T iPhone can surf the Internet at 3G speeds while a Verizon phone cannot, and that won't change until Verizon rolls out LTE and Apple adopts it, which won't be a while.
On the most recent Apple earnings call as of this writing, acting CEO Tim Cook said LTE forced Apple to make trade-offs they "weren't prepared to take." Translation: they drain the battery in a few hours. Every HTC Thunderbolt (Verizon 4G) user I've talked to has described the battery life as this: "it sucks." Word is it's good for about three or four hours of use.
So the iPhone will stay a 3G phone for at least another year if not two, and Verizon isn't upgrading the CDMA side. Then you have to consider things like coverage and data plans.