IPhone 6 Model Number Help.

I would like to purchase an iPhone 6 but need help with the model number (A1549, A1522, A1586, A1524). I am an AT&T customer in Minnesota, but will also need to use my phone in China. I am only in China for a short time, so it is more important that I get better service with my AT&T contract. I have a SIM card for both locations. (AT&T & China Mobile) and it would be great if I got good service in both areas of the world. Thanks.