Just get Straight Talk. You can buy a SIM package at any Walmart with 1 month service (unlimited data, calls texts), comes with regular, micro, and nano SIM cards for all GSM devices - you get SIM of all 3 sizes for AT&T and T-Mobile service, you get to choose which one to activate/use!
No contract, and if you already have your own unlocked phone, it's better than any contract service.