T-Mo is not going to unlock it for you unless you are or were a customer of theirs for at least 6 months and are in good standing. Know someone who is a T-Mo customer? Have them request an unlock for you. They only get 2/year for each line so make sure not to burn up any unlock they might need or see if they already reached their limit.
At a guess, the T-Mo has an extra 1700 MHz frequency enabled on the 3G/H+ radio that won't hurt it on AT&T. Also different carrier Apps.
If you are interested in SoftCard and Tap-to-Pay at all; you need to have a device branded to the network you are using in order to enable that system. So a T-Mo One (M8) will never get Tap-to-Pay on AT&T.