Late in the iPhone 5 model cycle, T-Mobile came on as an iPhone carrier. Apple updated the iPhone 5 models to include one with T-Mobile's fake "4G" (AWS) bands because T-Mobile never had 3G to begin with and went straight from EDGE to "4G" (AWS). On the earlier AT&T iPhone 5 models you'll get EDGE, 3G, and LTE, the later AT&T iPhone 5 models you'll get EDGE, 3G, "4G", and LTE.
Basically, you'll lose out on some sub-LTE speeds when not getting real LTE if you use your AT&T iPhone at T-Mobile depending in your iPhone 5 model.