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A1688 looks like "officially" the model you'd want, as it's the model sold by those carriers: http://www.apple.com/iphone/LTE/#iphone-6s-iphone-6s-plus

For LTE, T-Mobile uses bands 2, 4, and 12 and Free looks to use bands 3 & 7, which is actually supported on both models, so if you got an unlocked A1633, it should work, too. Older iPhones would have more bands missing. Other than that, for GSM/EDGE/3G use, both are also identical:
http://www.apple.com/iphone-6s/specs/
 
Actually in the US, you can only purchase the A1633 or the A1643. The good news is that Models A1633 and A1634 also support LTE networks listed for Models A1688 and A1687.

Source: http://www.apple.com/iphone/LTE/#iphone-6s-iphone-6s-plus

Dave

You meant A1634, not A1643, right? Thanks - I was a little foggy on that and only had the LTE band list to search from...good to know what's actually sold here! :)
 
I admit I didn't check the different model when buying my iPhone 6s Plus.
But then, I notice there is only one LTE band missing in my A1687, LTE band 30.
Hope I can use it happily at anywhere around the globe.
 
I admit I didn't check the different model when buying my iPhone 6s Plus.
But then, I notice there is only one LTE band missing in my A1687, LTE band 30.
Hope I can use it happily at anywhere around the globe.

Band 30 seems to be getting a very slow rollout in some areas on AT&T, but that's about it. It might be more important later on, but it seems that the 6s/6s Plus are the only devices in Apple's lineup that have it.
 
Actually in the US, you can only purchase the A1633 or the A1634. The good news is that Models A1633 and A1634 also support LTE networks listed for Models A1688 and A1687.

Source: http://www.apple.com/iphone/LTE/#iphone-6s-iphone-6s-plus

Dave

The Apple Store in the US will happily sell you a Verizon or T-Mobile A1688 model if you ask them. I just told the sales guy it was for use in the UK so he sold me a Verizon 6S and told me to throw the SIM away.
 
Band 30 seems to be getting a very slow rollout in some areas on AT&T, but that's about it. It might be more important later on, but it seems that the 6s/6s Plus are the only devices in Apple's lineup that have it.
And this band is for AT&T in USA only, for now.
 
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