I have both T-Mobile and AT&T. I travel to all 50 states evry year. I also travel to many countries. Very simple answer:
IT DEPENDS ON WHERE YOU PLAN TO USE THE PHONE. If you don't travel outside your home city. Fine get T-Mobile. If you want to talk outside the USA... T-Mobile is great (It is my overseas phone). But, the real truth is your can't get a T-mobile signal in about 85% of the USA. It just stinks. I mean really stinks. Let me be clear... USLESS.
Now, my AT&T phone is works EVERYWHERE domestic. Middle of nowhere...it works. But, completely uselss overseas*
*(except works in Hong Kong)