The factory-unlocked models, which start at $ 649 for the iPhone 6s and $ 749 for the 6s Plus, are models A1633 and A1634. Why is that interesting? These are the units AT&T is selling, and they include an extra frequency band that only AT&T uses.
The models sold by Sprint, T-Mobile, and Verizon are A1687 and A1688, which are otherwise identical to the AT&T units, but they do not include the AT&T-exclusive Band 30. Band 30 helps speed up LTE and reduces congestion in urban areas, and no other carrier on the planet uses Band 30.
If you buy a factory-unlocked unit, you get Band 30, so it will work optimally on all four U.S. networks, as well as pretty much every other carrier in the world. The unlocked phones are also eligible for Apple's installment payment plan.
There's no word on whether the unlocked devices use Samsung or TSMC processors, or, as is likely, a mix of the two. Apple commissioned two different versions of the A9 CPU, as AnandTech explains. While some people online say that the two versions have up to a 20 percent difference in battery life, Apple says the difference is more like 2-3 percent, and AnandTech says that the variance between chips even from the same manufacturer may make the whole argument pointless.