On Apple's website I have the choice of choosing my carrier (T-Mobile) or purchasing SIM-Free. They both say they're unlocked, but when I choose T-Mobile it says it comes with a T-Mobile SIM card. Can't I just use my current SIM-card? It just seems like unnecessary steps to choose my carrier first. Is there a benefit to choosing my carrier? Or is that mainly for people who want to switch carriers?
Are you paying full price?
If so, I'd go with the SIM-Free model and then just put in your old SIM.
It may say that T-Mobile will come unlocked (if paying full price) but YOU will have to contact T-Mobile to have it unlocked after paying for it.
Their policy as well is to have their phones active for no less than 40 days on their network before unlocking - so you may pay full price for the T-Mobile version but still have to wait 40 days before they will unlock.
With your old T-Mobile SIM that won't make a difference as you'll still be on T-Mobile's network, but if you try a different SIM before those 40 days are up the phone will still be locked.
Hope that makes it a little more clear.
PS. The technology differences are in the bands that T-Mobile uses for LTE and the LTE bands that the SIM free phone uses.