I am with ATT now (contract free). I like to order iPhone 6 contract free for ATT, but the only contract free phone now is Tmobile? I understand that I can use Tmobile for ATT and vice versa ?, but Are they the exactly the same phone ?
No. Yes. Yes. Buy AT&T phone for full price if you wish, it should be unlocked if bought from Apple store (walk in) and won't put you under contract.
You can by any device at full price at the apple store (except Sprint) and they all should be unlocked. I just purchased a Verizon 6 Plus full price and am using it on AT&T.