The following applies to 6s and 6s+:

*ANY* phone you can buy directly from Apple today is factory unlocked. That includes both carrier-specific models and no-carrier models. The only difference is that the carrier-specific models include the carrier's SIM in the box, and the AT&T-specific one supports an obscure AT&T-specific LTE band that you would barely miss as an AT&T customer, and which has no impact whatsoever in any other situation.

If you buy an iPhone from a US carrier other than Verizon, chances are it will be locked. I know from personal experience that T-Mobile's iPhone 6s and 6s+ are locked (but T-Mobile is pretty good about unlocking them).

Plus, a given model is the same price from Apple or any of the carriers. The only reasons to buy from a carrier are availability, deals or financing you might get based on your carrier's policies, etc.