So I got the Vizio P65 and the Samsung F8500 64", and the Samsung has the better picture, motion handling, colors, etc..., but mine had a pretty annoying buzzing coming from the set. It's not noticeable when music or dialogue is playing, but when it goes silent, you can hear it pretty easily. It's pretty much a deal breaker for me. I got both sets for around the same price = $2200. Note: I'm not using the TV speakers; I do have a pretty decent external speaker setup. I also sit pretty close to the TV ~ 7-8 feet.
I'm not a super professional viewer or anything like that, and I mainly just watch movies and tv shows, but for me, I'm going to return the Samsung and keep the Vizio. Honestly, for most people, unless you see the picture side by side, you're not going to notice that much of a picture quality/colors/etc. difference from these sets. Both are amazing for the price, and 4k does look really good on the Vizio. If you absolutely need inky blacks, then yeah, the Samsung is better, but just for watching TV shows/Movies, the Vizio handles blacks just fine.