I had a similar choice, but as it turned out the Samsung was an extra £200 (which I didn't have) and with those silly edge feet meant the TV wouldn't fit on my TV unit I ended up buying the 49 inch XD800 (specifically the XD8077). For the money (£585) it's been fantastic, HDR content looks incredible to me, incredibly vibrant, really 'pops'. 4k stuff looks amazing and the TV does a fantastic job of upscaling non 4k content. HDR seems bright enough to me, looking at the sun in the Witness or leaving a dark cave and entering a sunny area in uncharted 4 actually makes me have to squint. It's still a 10 bit panel so HDR performance is half decent. The wide colour gamut adds a subtle level of realism to the picture that o didn't think I would appreciate as much as I do, it's a much more natural coloured picture than my last Sony I had.
I will say that the blacks aren't the blackest, so if you play it in a very dark room it's going to look a bit greyer so the KS8000 might be a better choice. I play in a darkish room so it's not an issue for me. Grey uniformity isn't the best, you can really see the dirty screen effect when panning across solid colours for example in The Witness, but it's not an issue in scenes with more going on. Input lag in game mode is pretty low, 35ms; I have noticed a slight improvement in my Battlefield 1 performance, but not enough for me to be sure it's not just placebo.
Overall though, I've been extremely pleased with it, Ps4 pro games look amazing, uncharted 4 in HDR just blew my mind, Resi 7 looks amazing, the 4k stuff I've streamed looks incredible. For the money it's been a solid purchase. The KS8000 is doubtless the better TV but in the real world with my eyes and my disposable income, the x800 has been wonderful.