I've got the OP3 (before the T came out).
The 3T is a little bit faster (processor) and you have the option of a 128Gb version. The front (selfie) camera is slightly better, too.
Other than that it's the same as the OP3.
Updates AT LEAST once a month... currently on the latest Android 7.1.1 (you won't get that on a Samsung !)
NO bloatware either.
OnePlus have really got their act together after a shaky second phone (we won't talk about the OPX).
The best smartphone around for £400.