Horizon: Zero Dawn is a better open world game than Breath Of The Wild, in my opinion.
I did buy a Switch just for Zelda, and I thought it was a totally fine game, but I don't think it's a good Zelda game by any means. It's an open-world game with a Zelda theme, which is all well and good, but it isn't the Zelda game that I wanted.
Horizon, on the other hand, is the first game where to me the story reinforced the idea of the open-world structure. I enjoyed my time with Horizon much more than I did Zelda.
I wouldn't honestly advise buying a Switch just for Zelda and as someone who did exactly that, I hope you'll believe me when I say that right now it just isn't worth it. Wait for a sale, or until a couple more games come out for it.
Zelda is a much better open world than Horizon but Horizon does the RPG elements and Story much better. Zelda's open world is absolutely massive and a joy to explore. I'm not sure how you can say it didn't do the open world part better. Horizon on the other hand does combat and encounters much much better.