Up until this point, if you wanted to get the 3G version of the Nexus 7 in the US, it appeared that AT&T was the only option. Now, however, Google has provided a T-Mobile version on the Play Store. This model comes pre-loaded with a T-Mo SIM. According to the tech specs, though, they're functionally identical. This will just save you the trouble if you prefer magenta over blue and orange, I suppose.
The price is exactly the same and even the radio bands are identical, so regardless of which unit you buy, you should be getting the same hardware.