T-Mobile really doesn't save you any money. Their plans are just about as much as AT&T but with lesser coverage. However, when I dealt with T-Mobile customer service, they are certainly better than what you would get from AT&T or especially Verizon!
T-Mobile, calls don't come in, even with service.
AT&T, Calls come in, but then drop, GSM
Verizon, Horrid customer service that doesn't help you at all. Also CDMA
Sprint, Worse customer service than Verizon!
Theres always lowend carriers like Cricket, which has no smartphones worthwhile.
Take your pick! I am with AT&T, and I am just going to stay with them.