My biggest question is, if I buy this phone today, with AT&T allow it on their network? My gut says no, they've been brandishing an ugly whitelist around for their big LTE push in February...
Unfortunately T-Mobile isn't a great option for this phone either, because the phone lacks band 71 (T-Mobile's only nationwide low-band). It may work well in some areas, but have no coverage in others. Verizon has a sort of pseudo-whitelist; you probably will have to activate a whitelisted phone and then swap the SIM, but it may work.
https://www.att.com/idpassets/images/support/wireless/Device...