I realize this is also being discussed in
this thread, but this one's more on point.
I understand that the AT&T and T-Mobile iPhones are identical and will work on either carrier if they're unlocked. But Apple and AT&T reps both told me that if I buy a T-Mobile 5s from Apple and try to use it on AT&T's network, data transmission may be degraded. I don't believe that's true, but it's hard to know for sure if it's worth the risk.