Are the Samsung UE65MU6400 and LG 60SJ850V good models and if so, which one to choose?
The 6400 is good and great vfm IMO; I had one before upgrading to the Q7. I have no experience of the 850 so can't really comment. I have a 960 (which has just been upgraded to a ZD main board to make it HDR compliant) that I use with a PS4 Pro and GT Sport looks great on it.
The 6400 is only 8-bit HDR and doesn't have the brightness performance of the Q7 which is 10-bit HDR.
I was using the 6400 with an XBOS and found that on FM3 the HDR only seemed to work 50% of the time for a reason that I never got to the bottom of.
When I was looking for something better to use with the X.B.O.X I went to an A/V show and after talking to the Samsung guys, the Q7 was the one that they recommended - it was one of the brightest and sharpest TV's in the room.
So far I'm very happy with the way FM7 looks and runs on the Q7 in 4k HDR; I haven't tried FM3 yet as I'm waiting for the upgrade patch in Jan.
Proper HDR on a game that supports it is well worth having IMO and it's worth paying the extra for a TV that will run HDR in all it's glory.
As an example, in FM7 the colours and lighting of the sky as dawn breaks in an endurance race looks amazing. The bonnet reflections at night and the rain/lightning effects. When racing in the Bernese Alps there's wisps of clouds between the mountains that I never noticed before in FM6. In Dubai you get the sand blowing across the road. It all adds up to a great experience, which once that you've tried it you won't want to be without.
I hope this helps with making a decision.