tl;dr:
If, as a customer/consumer, you are looking to maximize your iPhone 6s/Plus experience
while using it with your carrier in the US, ordering the model offered by your carrier will be the safest bet.
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From the info that's out there now, it's pretty much safe to assume that AT&T is offering the A163x models since they're the only carrier in the US (
at this time) that offers LTE Band 30 to supplement the other LTE Bands that AT&T uses.
It's also pretty clear that Verizon and Sprint will get the A168x models in order to have CDMA signal fallback should the LTE signal be insufficient.
The question would be which model T-Mo is offering. T-Mo subscribers can't go wrong with either the A163x or A168x models as T-Mo operates within the GSM and LTE bands that are currently listed on the
Apple spec page for both flavors. Also, since LTE Band 12 (T-Mo's 700MHz) is now included on both flavors, T-Mo customers will have better coverage where Band 12 is offered (compared to iPhone 6).
Is that concrete info that the AT&T and T-Mobile phones will be the exact same model?
There's no concrete info as the units aren't shipping yet. It's also a tossup, since there's a published PCMag article without much of a citation as to where the info about T-mo offering A168x models came from vs. a tweet from an official T-Mo account explicitly stating T-Mo is offering the A163x model.
Also, I'm sure there are both financial and regulatory reasons, but curious why Apple doesn't just make one single model.
I'm certain Apple would like that, since they could have a single SKU each for 6/Plus and streamline the whole manufacturing process.
Update:
Checking the Apple Store app on my phone, it shows CDMA when I select Sprint or Verizon (obviously), and GSM when I select AT&T. When I select Contract-Free/T-Mobile, it also shows GSM, which leads me to believe that the T-Mo phone will also be the A163x models.
Note that the T-Mo iPhone 6/Plus models at pre-order time last year were also the AT&T models (A1549)