The new iPhone 6s and 6s Plus each come in two models that work on global LTE networks; a third set caters to China’s unique network demands. Some confusion resulted from Apple’s update to its LTE compatibility page, which seemed to imply that the model for AT&T’s network wouldn’t work over LTE on Verizon, T-Mobile, and Sprint’s networks. But that’s not the case, as Apple confirmed for Macworld.
Buy any non-Chinese model, and it will work over LTE (and regular networks) on any American carrier. There’s just one small difference with AT&T involving additional frequencies it has a long-term plan to deploy fully.
The confusion arises in part because the explanation and split among supported networks is somewhat different with the iPhone 6 and 6 Plus. With the 2014 phones, the model oriented toward AT&T, T-Mobile, and Verizon lacked some bands primarily used elsewhere in the world. That’s been resolved in the 6s/6s Plus, and Apple could be clearer about communicating it.