Just talking on a sheer graphics level, I'm not sure why people would bother comparing BoTW and HZD. One is a Wii U game, made on/for Wii U hardware, ported to Switch with slightly better resoluation/framerate. Slightly.
Horizon is a game made on PS4 hardware, which is MUCH more powerful than Wii U's, obviously. It's even more powerful than Switch, because they decided to spend extra money on pointless bells and whistles ("HD Rumble", etc) in the controllers, than to have even somewhat stronger hardware. However, Switch hardware IS still more powerful than Wii U, and a Zelda game made from the ground up on Switch, which I absolutely hope to see, WOULD look a lot better than BoTW.
It does come down to art style, animation, details, things like that, and BoTW does all that very well. But speaking on a purely graphical fidelity level, there are also certain areas where, as beautiful a game as it is (especially when it comes to sprawling grassy fields, or rain downpours, etc.), there is altogether TOO much haze/fog in the game, which they obviously did to help the framerate with such a massive world. To ME, on stronger hardware, I'd be able to see MUCH more of the horizon CLEARLY, or be able to see down into MORE valleys and sprawling areas from on high, without seeing everything be covered in a thick haze. There are times that honestly really bothers me. BoTW also has a serious issue sometimes with pop-up, not being able to see certain enemies, etc. from far away, until you get close enough and they just "pop" into view finally, like giant Hynox for example. I should be able to see enemies from far away as I'm sailing in that general direction, so I can better plan my decent, etc.
Granted, BotW is a brilliant game in most aspects, and certainly well made. But it's hardly perfect, and IMO, could STILL use a lot more polish than it already admittedly has. I would love to see another crack at such a huge, vibrant world, but to do go the rest of the way, so to speak. I don't necessarily want an art style that is EXACTLY like BoTW, though that is preferable to me over a more Wind Waker look. I think something a bit more around that Wii U tech demo would be nice, just with more color.
I would 100% also like to see them keep manual jumping and rampant climbing, but I'd like to see the jumping more integrated, more useful in and of itself. Such as, for example, using it to have more aerial attacking abilities, or use it more in certain puzzles, maybe being able to "wall hop" or run along/up walls a bit, etc. Just little things like that, could add an insane amount of depth to combat and puzzling alike.
In general, I love BotW. I still haven't beaten it, so I'm not fully convinced that it's taken my "Fav. 3D Zelda" spot away from TP just yet. Though I do love the lush/expansive land.
As for Horizon, I think it looks cool, though while it does seem to have certain advantages over Zelda, I'm not sure that Zelda DOESN'T have, overall, more content going on in the game world. Then again, to be fair, I have not yet played Horizon.