Last year the difference between unlocked and sim free was Verizon and sprint not liking your imei when you tried to activate service
Verizon on the tmo last year wouldn't allow you to activate it. You had to use a SIM card from another phone. And sprint would flat out not work.
When the sim free came out Verizon and sprint liked the seriial number. A few weeks ago Verizon now likes both T-Mobile unlocked and sim free
When I try to go to Verizon or sprint and ask them about my tmo unlocked iPhone 6s Plus they try and tell me I don't have a compatible device.
ok so the sim free is the better option in terms of being free and clear of any carrier. I thought so and it makes sense.
I've tried to get a Tmo unlocked phone to work with straight talk and have had issues. Meanwhile my friend bought the same phone from straight talk (probably a sim-free model) and his phone works on straight talk no issues.