I had AT&T, and Verizon's signals are much better (can you hear me now?).
I have had AT&T for 3 1/2 years. They used to be a great company, although you DO pay through the nose for the customer service.
Lately, customer service has not been so good. In fact, it is BAD.
Here's something I observed: I can be driving by a cell tower, and lose my call. I found this curious, so I began asking some other tech heads about this. Apparently, AT&T uses the TDMA service method, and Verizon uses CDMA, which is not compatible. Those towers I was passing were likely CDMA towers, and there are more of them. That means better coverage for Verizon and anyone else that uses CDMA, and not as good for AT&T or other TDMA carriers.
AT&T is going to be adding GSM as the new service standard, and eventually will drop TDMA. Verizon may add GSM, too. That will take several years, and I don't recommend switching to a GSM phone for at least three more years, unless you use your phone in Europe, where GSM is the predominant standard.
My service agreement with AT&T goes up in January, and I will likely go to Verizon, which my friends have. They LOVE their customer service and rarely have a problem. Recently, my friend had a problem with their phone and got a Samsung A320 phone free. When I had a problem, AT&T was going to make me BUY a new phone, one of those crappy miniature Nokias, if it turned out to be a phone problem instead of a system problem.
I'll take my phone number to Verizon and call myself "wiser" than I was before.