I used to sell TVs years ago and knew just about everything about the market. Then I quit and erased most of it from my memory because I hated it.
Vizio: I don't know quite as much about these as others (my store didn't stock Vizio for some reason, even though we carried Sony), but last I knew, they're Sony's bargain brand. If you want a cheap 4K, Vizios are probably your best option. I've heard people hate on Vizios, but my family had one for close to a decade that never really had any issues. Picture seems to be a little more washed out and not as vibrant as the top companies from what I've seen, but they're probably the top of the cheaper brands.
As far as gaming and graphics goes, there's not much difference between the two. It's console plebbery, they're running cheapie hardware, typically running games at 30fps, you know the drill. Xbone has backwards compatibility like you mentioned, and they also give out a handful of free games every month (including some pretty great 360 titles), which is pretty cool. Sony has become kind of an asshole company who doesn't listen to their player base at all. Even the Wii U is backwards compatible, Sony just straight up didn't bother with backwards compatibility because they'd rather you buy shit again on PSN.
It pretty much comes down to backward compatibility, exclusives, and the 4K drive. As far as exclusives go, Sony wins a bit there, but barely. Uncharted has run its course, God of War is hardly a reason to buy a PS4, and the old classics like Spyro and Crash have all but faded into obscurity. PS4 has some cooler weeb games if you're into that sort of thing, but besides that there aren't many worthwhile exclusives on either Xbone or PS4 these days (Halo has turned to crap more or less as well). Nintendo retains its crown as the weak gimmick console company, but they're got the best exclusives by a wide margin. Honestly Nintendo consoles are the only ones worth buying for exclusives.
I'd lean towards the Xbone, honestly. PS4 just doesn't have a lot going for it in comparison.