Origins is more polished on some details.
But Odyssey has a better replayability

Both are excellent IMO, Odyssey builds on Origin's foundations and has way more stuff to do (which I find brilliant, others don't like it). Sea Battles too (akin to Black flag, but I miss the telescope for identifying targets). Personally I think the storyline of Origins is better (although it's a well worn story) the animation is better in Origins too especially in cutscense. Alexios (only played male so far) facial expressions can go a bit weird when choosing replies.

I prefer the main character Bayek in origins to Alexios in Odyssey. I think the opening act story is better in Origins, but they both go a little off the rails with the alien tech stuff towards the end. The combat in Odyssey is flashier because of the ranged takedown mechanics they added. You should also consider which setting is more interesting. I prefer Egypt to Greece, but you might feel the exact opposite.

I wouldn't buy both together and suggest a long break between. The core gameplay is really similar and Ubisoft titles can get repetitive quickly.