If you keep the AT&T one, go back to WalMart and buy the Straight Talk Bring Your Own Phone activation kit for $60, which includes your first month service. So it is 15 bucks for the SIM card and $45 for your normal month basically. Straight Talk plans right now offer the same "unlimited" yet metered data plans as Tmobile. Which means you get 3GB of data at full speed (my 920 gets LTE), and then they throttle you down to 2G after that, but you still have internet access. You also get unlimited talk and text. This is all for $45 a month, pay as you go.
So basically the price should be the same if not similar, but you would be on a much more robust network.