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