If you don't mind me asking, when you talk about TW3's horses being better, are you referring to the animation? I saw footage and I thought it looks a lot better than DA:I's approach.
I'm weirdly into horses in RPG games for immersion sake, I don't think saying TW3's horses are strictly better is accurate. DAI's horse model is better, TW3's horses seem to look like they're going to fall forward, perhaps its the front legs being in a weird place. I prefer DAI's walking / trotting / cantering animations because of it (horse never actually galloped QQ). TW3 also doesn't have a rear option, which is a small thing but its almost a given in every other RPG with horses, so thats sad.
DAI's colourings are superior, with the horses in TW3 (except the brown ones) looking a little off with few variations.
However, Witcher 3's idle animations are faaar superior, and the fact you can customise your horse (blinders, saddles, saddlebags) add to it. Your horse in TW3 also seems to have personality, will wander off to eat apples or find a water source to drink, which is a really nice touch. He seems to ignore you if he's eating and you whistle for him, don't know if thats intended but I like it 
Not disappearing into a cloud of smoke is an added bonus, with that being really annoying in DAI for immersion's sake. And the ability to fight on the horse (and I LOVE the slow mo) is much appreciated.
TLDR; If the two merged they would be the ultimate RPG horse, and I probably enjoy horses in RPGs way more than the normal person ;_;