If you care about getting updates from OnePlus including getting into the beta update program, don't get the Tmobile version. Even if you unlock the phone, the T-Mobile version won't support that. To me it's a pretty big deal. The T-mobile version isn't even running the latest version of their OS and nobody knows when it will be released.
I'd buy the phone directly from OnePlus.
Myself, I just put my SIM back into my 6 and I'm gonna return the 6t this week. I may wait until they announce some new colors and rethink opting for it....or just wait for the 7.