It all depends (doesn't always). If T-Mobile has good coverage where you live and work and you don't travel much then T-Mobile is cheaper and the service seems to work just fine. If you don't have very good coverage where you live and work or if you travel a fair bit then T-Mobile will probably just be a headache for you. For the Verizon posters, this is an AT&T sub-forum (you have your own) and the OP question was T-Mobile vs AT&T. Just say'n.
Mike