There are a number of reasons for going with T-Mobile:
Fantastic International call and data rates
Unlimited Data streaming of some music sites
no contract
However I'm dumping my unlimited AT&T for Consumer Cellular and buying the GSM no contract T-Mobile phone. Consumer Cellular for me is better because:
Uses AT&T Network (better than T-Mobile in my travels)
No contract. If you need to temporarily upgrade you can on the spot upgrade to more data or calls, then downgrade immediately when you don't need them.
Cheaper. 600 minutes, 1 GB is $40 a month vs $50 with T-Mobile (although T-Mobile has unlimited talk, throttles data after 1 GB)
Top Rated by Consumer Reports. And I can confirm this as I had to call AT&T, T-Mobile, and Consumer Cellular support today. Consumer Cellular was an order of magnitude better than AT&T and T-Mobile.