Our company has a deal with AT&T back in Dallas but most people get Verizon phones out here cause of reception issues. I even have 4G down in the field office I work at. Don't know anyone with T-Mobile. If you are just looking to save money, you could look at Aio Wireless...AT&T's network but with no contract. No phone subsidies but the plans are cheap and you can use any AT&T compatible smartphone. Depending on how much data and stuff you use, it might be cheaper to buy the phone you want outright and pay less per month.
http://www.aiowireless.com/shop/plans.html