Thanks for the info. My main concern is unlimited messaging and data. I don't know how T-Mobile can offer more for half what AT&T costs. It's crazy how much cheaper T-Mobile is.
T-Mobile is definitely cheaper. Since you can get a good signal in your area, it's better to use them instead of AT&T. In fact, I've only heard of 3 advantages for AT&T:
1. AT&T's coverage may be greater in some areas. This may not affect you now, but if you decide to travel or go on vacation, you may find yourself in an area with no T-Mobile signal.
2. AT&T's signal may penetrate buildings better. This is important if you work inside a big office building, for example.
3. AT&T has conditional call forwarding. This is important for using some features of Google Voice. If you don't use Google Voice, you don't have to worry about it.
The beauty is that both T-Mobile and AT&T have prepaid, no contract plans. So, if you're going on a trip in an area with no T-Mobile signal, you can just sign up for AT&T temporarily and use that for your vacation. Once you return from your trip, you can go back to T-Mobile.