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.