What is the advantage to buying a carrier-specific iPhone over a sim-free version?
Apple sells carrier-specific iPhone models so that the carrier can do subsidizing or financing. The carrier-specific ones comes with a pre-activated SIM for the carrier that you choose. There is no advantage other than convenience when paying in full (other than the Sprint requirement). One thing, though, is that the SIM-free version doesn't come with a SIM, so you need to get one from you carrier, which might cost money. The SIM-free and T-Mobile versions come factory unlocked, and the others might not.