just get the unlocked fully paid for model. t-mobile and verizon and sprint are selling the exact same model.
the AT&T model is exactly the same as those 3 , but it has 1 extra lte channel
the only complication your going to have are phone companies like sprint and verizon telling you they either won't activate or guarantee to activate the device because it doesn't show up on their lists of devices.
I havnet actually tried buying verizon or sprint on my phone, but asking them to check my meid is making it sound like they cant activate it
if you want the official apple word. go to http://www.apple.com/iphone/LTE/ and look up what country you are going to and which carrier, and that chart will tell you which model to get