One thing that really bothered me in Origins is how you solved everything with violence.
Normally this is not something I'd care about but Origins was really extreme about it. The most insignificant conflict you find in a sidequest? Bayek pulls his sword. In Odyssey, this is greatly reduced due to the dialouge system and general ability to make choices. Although, it's still far from perfect, but a lot better.
I voted Odyssey, i think it's the better and more fun game to play -- I preferred Odyssey's faster, more arcadey combat and I definitely preferred Odyssey's storyline. I was way more invested in it. Although Odyssey gets a huge minus point for the shitty split up ending structure. I think Odyssey's ending, if you combine all of them, would've been absolutely fantastic. But split up like this? Ehhh.... very strange indeed. Not to mention that if you do them in the "wrong" order they can be quite jarring too, especially with characters not acknowledging if you completed an ending. That's just really dumb. And I bet most people will just stop after seeing one ending. And I can't imagine how underwhleming that would be.
But, Odyssey's modern day actually felt interesting for a change (as opposed to Origins' nothingburger) and there were a lot of characters that I actually cared about and liked. In Origins, I honestly only liked Bayek and Aya, and maybe considered Cleopatra somewhat interesting, but that's about it. The middle portion of Origins was full of characters that I was meant to care for, given how Bayek was apparently solving everyone's personal problems, but... yeah. No. Odyssey still has this issue to an extent, but not much more than majority of RPG's, I think. It's way better, especially with the dialouge choices that often allow you to just tell people to sod off.
Origins had a few side quests I liked but most of them were just opportunities for XP. Odyssey, I'd say, most side quests I did, were good to great. There were some quite funny ones too, which was a nice change of pace. I also liked how some sidequests would escalate, which reminded me of the Witcher 3 a bit, where you could be doing a side quest that at first seems utterly simplistic and basic, but soon evolves into something much more interesting.
The one thing that Odyssey let me down with though is the world. Sure it's beautiful and detailed, but nothing comes close to the pyramids or Alexandria in it. Athens felt especially underwhelming, it just looked like any other city in the game really. The tombs in Origins were also genuinely fun to explore, whereas Odyssey ones are... err....they reminded me of FF15's dungeons. Just really basic layouts and almost as if they were randomly generated, with only a handful of assets. The way they spammed snakes on every corner as well-.... like god damn. It doesn't get any more "game-y" than that.
Overall, I expected Odyssey to be "Origins but better in every way". It's not that, but it's close enough.