Interesting thread because I got Zelda at launch, completed it around the beginning of April, and then waited a couple of months to start Horizon and am playing through it now.
People are right when they say they're very different but for my tastes Zelda is significantly more appealing. A 10/10 compared to a 7/10.
Horizon certainly isn't a bad game but it's like the ultimate refinement of the Ubisoft template. Despite the open world trappings it's quite guided and cinematic, and you'll be spending a lot of time following icons on your map. The focus is much more on combat and story rather than exploration. Exploration doesn't really reward you in Horizon anywhere near as much as in Zelda (a big part of this is Zelda not putting icons on your map, so everything you discover feels earned).
It's a good game but, even leaving a couple of months between Zelda and Horizon, the latter now feels quite old-fashioned to me, despite the graphics. It's gorgeous and combat is slick but apart from the setting it's very been there, done that.
Zelda is much more about puzzles, exploration and interacting with the world and its physics. Now personally, that's much more to my tastes. Exploration feels organic and there's always several things in the distance pulling your attention this way and that. Gameplay variety is unequivocally much greater. The story is weak but that's just a frame to give you an end goal.
The exploration element in Zelda had kind of raised the bar on what I expect from an open-world game so it's taken me some time to adjust to going back to the standard template. The world design in Zelda is so outstanding in terms of guiding your attention and giving you incentive to explore that going back to the icon following style of game is tough and initially disappointing.
Really though, my experiences with both games can probably be summed up by this:
I'm playing Horizon right now and I play it for maybe 2-3 hours per session. When I'm not playing I don't really think it about it or what I'm going to do in it next time I play. It's enjoyable but it hasn't consumed me in any way.
BotW on the other hand, completely and utterly consumed me. I would wake up early before work to get an extra hour in. A quick session would turn into 10 hours in a flash. I'd skip lunch to carry on playing. When I wasn't playing, I was thinking about playing it. When I was at work I was planning what I was going to do in a game once I got home.
It's years since a game did that to me. Probably going back to Dark Souls 1. Just one of those games that completely redefines what you expect out of a genre. I can see why people might prefer Horizon if they want a game that's more structured and cinematic and combat focussed, and that's totally fine. Zelda just resonated so much more with me.