I would not recommend it. While TMobile is vastly more affordable than ATT (Just by looking at the plans. Personally, I'm on Verizon), what you get in $$$ savings is off-set by T-Mobile notoriously horrible building penetration and spotty high-speed data coverage. You're moving from a tier 1 wireless carrier (ATT and Verizon) to a Tier 2 carrier (T-Mobile & Sprint) and you can anticipate making some sacrifices in service.
Ultimately, it depends on what is most important to you? Do you want reliable voice AND data coverage? Then stay with ATT. They tend to be pricier, but they leverage their reliability and coverage to rationalize the premium. Do you expect to use more data or are in fear of overages?- Then go to T-Mobile. But T-Mobile lacks a lot of the sub 700MHZ spectrum, so you'll never really get good data penetration in buildings.
If you want to save a few bucks, take a look at Cricket Wireless, it's the subsidiary of ATT and uses the ATT network. Prepaid with lower rates and what not. Good luck!