Apple iPhone

Hi I am about to buy a new iphone for T-mobile. People are selling them unlocked for 650 bux online for the 16GB + shipping!?!@ Thats ridiculous considering you can go to any att store and buy the thing brand new for 500, no waiting for shipping and download software to unlock it for 40 bux, maybe cheaper. Anyway, when you buy it in the store its the lastest firmware of 1.1.4, I may buy it online if there is a better firmware or that has different features? any input is appreciated