Interesting that GG was able to make a game on a scale of Witcher 3 with less people and a smaller budget! I also like that Horizon has much more variety in environments, from snowey mountains to forests to grand canyon type areas. W3 felt almost the same the whole game until Blood and Wine.
I do not think this is close to a comparison here...
I have the game (and played it all day yesterday) and yes, Horizon Dawn is a very good game (I really like it); but you cannot compare it to the Witcher 3 in term of scale. While the map is huge, everything is just not at the same level than Witcher 3; let's be honest.
Some Horizon concepts are closer (in term of simplicity) to Tomb Raider for instance (the crafting system, the map and location diversity, story and dialogs, etc...) while the Witcher 3 is way more detailled and defined in all areas. Only area I would put Horizon first (but this is based on my taste) are the combat, I really like it (the traps, the cable stuff, etc..) this is really cool to play. Not even mentionning the Witcher 3 is multiplatform.
I'm not a big fan of the Witcher 3 (and I really like Horizon) but you cannot honestly say that the scale of the game is the same... You can see the difference in budget when you play both games. I'm not surprised about the budget being almost half of the Witcher 3 and you can see it in the game.
All in one, Horizon is a great awesome game, defintely an highlight of the Play Station Four imo. This is the game that made me play with my Play Station Four more than 1 hour after all :)