Okay, so the Verizon iPhone 4 and the AT&T iPhone 4 have pretty much the same hardware, aside from Verizon's CDMA network not being able to handle simultaneous voice and data and a few internal tweaks. But there are some less-publicized user experience differences, too.
Specifically: while A&T's version can handle up to five calls at once in conference mode, Verizon's can only work two. The Verizon iPhone can't put a call on hold, and requires a bit more effort to activate call forwarding, call waiting, and caller ID features. That's probably not enough reason to choose one over the other—in fact, with iPhone 5 just around the bend, you should choose neither!—but hey, the more you know. [Apple]