How do you plan on connecting to the internet? AT&T offers DSL for the home, but I don't know about T-Mobile -- do they only offer mobile internet services, or can they install DSL or some other internet service in your home as well?
Also, I'm sure each provider offers different tiers of service as well... where I live, you can get AT&T DSL in 1.5Mbit, 3.0Mbit, and 6.0Mbit downstream services. Obviously, the higher the number, the faster the internet connection... so saying "AT&T" doesn't give any clue as to how fast their internet is, since they more than likely offer different speed services that you can choose from.
I like AT&T DSL. I have 3.0Mbit DSL service and it's flawless. It's fast enough for my needs. I had 6.0Mbit DSL service at one point, but due to moving, they didn't offer that speed service in my new area (just like what others have been saying in this thread -- exactly where you live makes a HUGE difference in what services you can get).
In short, it's really not WHO you choose to go with, but which tier/speed service plan you choose from the provider. If AT&T and T-Mobile both offered 6.0Mbit DSL service, it really doesn't matter which provider I choose -- they're both the same relative speed.
At any rate, I feel I've gone on a rant about home-based internet services, and you're probably looking for mobile-based internet services. In that case, it would be prudent to call each provider and ask what their transfer rate is for their mobile-based internet services. Whichever one tells you the higher number is the winner.