Hard to compare, really. Witcher 3's plot was weak and ploddy IMO for supposedly the best RPG ever and the combat/animations/movement were rather poor.
Graphics were very uneven.
On the other hand, the world was nice and some sidequests were great. Gwent was pretty addictive. Also, the expansions were meaty and had interesting storylines. You could also choose your dialogue options to change the outcome of certain events, which you can't do in Origins.
Origins has a more impressive, vivid world that feels alive. The cities, landscapes and details kill Witcher. Question marks are better and more varied, you can find lots of amazing places, tombs, camps etc. with their own little stories told through old manuscripts. The mechanics, horse riding etc. are better and you can climb everywhere which makes the world feel even more massive.
Overall, world, gameplay, movement and graphics go to Origins. The rest is preference, as the combat can be a mixed bag and the story isn't anything amazing, especially that W3 gives you dialogue choices and has better side quests (I think). Nonetheless, sidequests in Origins are still very fun and meaty, they fit the game pretty well.
I prefer Origins overall, because I find the setting and movement to be more engaging and open. Ancient Egypt just feels fresher and more unique than your typical medieval RPG setting.