there is a difference between Sim Free and Factory Unlocked...
Sim-Free will get it with no SIM, but whatever SIM carrier you pick, will get locked to that.
Factory Unlocked means Factory Unlocked, whatever SIM you pop into it, will work.
Factory Unlock for me always, I don't want to be tight to 1 carrier if I decided to change at some point, and it is cheaper to get Sim-Only plan (this is in the UK at least).