I agree with the others.
T-mobile got in the US market by acquiring Voicestream - and usually Voicestream's allocation of spectrum puts it in a range that doesn't do well in buildings (~1900Mhz). AT&T, on the other hand, has a larger number of towers and a usually uses a section of spectrum that penetrates walls much better (~850MHz). They both usually share towers, and AT&T has a lot of 1900MHz towers - but overall, T-Mobile tends to be on the high frequency end of the spectrum, while AT&T is on the lower.
On the other hand, T-Mobile tends to be a bit cheaper. And I personally think that T-Mobiles customer support is much better than AT&T's - having used both for a quite a while.
As others said, if you live in a metro area, then T-Mobile will probably work well enough, and be cheaper, and in my experience they are much friendlier. If you are not in a place with a lot of towers, then AT&T's better section of spectrum will probably result in better signals and less dropped calls.