As most of you know, there's a big difference between an unlocked bootloader & and unlocked "device", (or sim unlock) that you're able to use on any carrier. As long as your buying a GSM branded model, you'll be able to get an unlock code to use it on most any other GSM carrier's network. Obviously CDMA models (Verizon/Sprint) somewhat limit this ability, but many of the newer flagship models are providing more GSM bands that will allow the device to work on other carriers once unlocked. I read somewhere recently that the Verizon S6 will allow this, though I'm not sure this is confirmed. I'll have to double check.
S5 tap'n