I have had both carriers (AT&T when it was Houston Cellular. Switched to T-Mobile right after the name change from VoiceStream, in 2003). I have had coverage issues with T-Mo that were easily solved by replacing the phone and/or SIM card. I never had any problems on Houston Cellular. Both carriers work fine in my house and area. My parents are still currently on AT&T so I know it still works.
The deal sealer for me is that T-Mobile is WAY cheaper than AT&T as a college student. I was on a family plan with my parents on Houston Cellular so I never saw my portion of the bill LOL
HOWEVER, once I start working, I may switch due to the fact that AT&T gets phones I want. I have my eye on a certain AT&T phone but I'm not forking out $300 for an unlocked GSM version. Also, the data and voice prices are slightly higher but I will be able to afford it.
Just my take on the 2 carriers. Who knows, I may be able to afford one phone on each carrier LOL