The D851 is the Tmobile phone and will have all the Tmobile features and full 4G access, Wifi Calling, access to Tmobile account. If you want to use on Tmobile pre/post paid get the D851.
The D855 is international version, factory unlocked, been reading you wont get full 4G access no matter how much you play around with APN settings.
Factory unlocked D851's are hard to find, many people, most sell with unpaid bills and do not have clean ESN/IMEI, plus it will always be tied to seller so you need to trust them, as they can report it stolen or lost etc after they sell you the phone. They need to have $0 balance with Tmobile and pay off phone to be unlocked.