With iPhones, it's possible to just swap SIMs if you're on AT&T, T-Mobile, and sometimes Verizon. Sprint is a dick and makes you contact them to activate any new phone you put the SIM into
If the AT&T model is different I'm going to be so mad. I have to activate on AT&T first just to get the phone, then cancel that line and move to T-Mobile. I wonder if I can just tell them "Yo, I'm doing the iPhone Upgrade Plan under AT&T right now, but give me a T-Mobile phone" The difference is going to be in the bands, the AT&T phone has LTE Band 30 but none of the CDMA ones, the T-Mobile phone (based on what others have said in this thread) has all the LTE bands (minus 30) and all the CDMA bands, which makes it more attractive
Where'd you get this info?