I moved down to miami/ft lauderdale with AT&T and will never go back to them. Constant call drops and yes you may have a signal but you probably won't be able to use it much. Verizon coverage inside buildings is not great but at least I know that when I have a signal it's going to work. No way I'm going back to AT&T. You also have to consider if you're going to be in a high tourist area because they all bog down the AT&T network. I'm on the beach and sometimes I would drop a call 2 or 3 times in the span of a few minutes.
I've been nothing but happy with Verizon.