To get a carrier specific iPhone, don't you have to agree to their contract?
For unlocked you can get either option below without contract.
What I'm saying is. From my chat with someone on the Apple website.
The only difference between Sim-Free and T-Mobile is that the T-Mobile unlocked version comes with a nano sim you can use to activate the phone.
So you can just get that version for 9/20 delivery unless I was given wrong info.