I bought the T325 and I am using it on AT&T. Unfortunately, it doesn't have the correct LTE bands for the U.S., so the best it can get is H+ which is 3g. Having said that, I don't have LTE where I live but I get 14Mbps on H+ so I'm not missing the LTE at home. I travel a lot and most of the places I go to have 3g speeds between 3-6Mbps so it does ok (a lot better than hotel wifi).
The thing to consider is that on AT&T, and I'm assuming t-Mobile as well, the T325 shows up as a cell phone so it will require a full cell phone sim. A data-only sim won't activate. I also had to tweak the APN to get a solid connection.
All said, I'm very happy with the T325 and I use it as my daily phone/tablet.