Pricewise, I see no reason to switch. Both prices are about the same, both have Friends and Family/A-List, and AT&T has rollover, so +1 for them. (If anything, Sprint would be much more tempting in this regard, there's good coverage where I live, and the plan prices are way cheaper)
The biggest problem right now with AT&T is the iPhone. One network simply can't handle the data traffic it creates, and over the past couple years since the iPhone's release, I think AT&T's network has shown a lot of improvement, just not enough to keep up with the iPhone.
So, to answer the question, if there was a Verizon iPhone, I think I would wait a month or two before I decide. I think there's a big potential that enough people would leave AT&T for Verizon, and in turn take a lot of stress off AT&T's network, leaving the people who stick with AT&T with a much more capable network.
If, however, AT&T still can't cut it and Verizon ends up showing that it can, then sure I'd switch, but I don't see that as likely to happen.