Origins was pretty good. It was flawed because this was obviously their first attempt at RPG mechanics and they were just dipping their toes into it, but overall I enjoyed it. However, I put it down and never picked it up again once I got into Breath of the Wild. So yeah, it's definitely one of those games to try only when you have nothing else to play.
The character perks/talent trees, if you will, were extremely shallow in that department, so leveling was kind of pointless. Odyssey is way more fleshed out in that regard and I absolutely love the "hot key" system they have now. Also they added in scaling so you aren't massively over leveled mid way through the game like you were in Origins, and that is a huge help. I'm 90 hours and counting (about 2/3 of the way through the main story according to trophies), and it's not trivialized yet.
The story was eh, but honestly I feel the same way about Odyssey. They really have a horrible habit of trying to make ancient world people talk like someone from the inner city in 2018. Also they really have a revisionist view of history in both games, and at times it's kind of eye rolling. However, Odyssey has much more cinematic side quests. Most regions in Odyssey have their own self contained mini stories that are very well fleshed out. Origin's character has 0 personality or charisma and frankly he was a bore, versus Alexios/Kassandra who both have their unique bit of flare that makes them way more entertaining. If story is your primary pillar in playing, I'd say Origins will grate you.
Origins does have a form of Mercenary yes, but it's way more static and finite. There are like one or two of them wandering each region, and if a flame is lit or you make too much of a scene they will start b lining to where you are. They are MUCH harder than mercenaries in Odyssey but once you down them they don't get replaced. And it's only one so you can track where they go and go to the opposite side of the region and go on a murdering spree, and bug out before they show up. Personally, while I enjoyed what Origins had, I have had a blast with Mercenaries in Odyssey just due to the fact that they make situations so unpredictable due to 3 or 4 Mercenaries interrupting your massacre of a fort.
The main thing I think Origins has over Odyssey is it's a much more focused game. You are pretty much 3rd person guy making his way through Egypt taking out forts, camps, and other places full of bad guys. It had some forced ship missions, and the crappy modern day parts, and it was jarring to be in the midst of your goals to only have to stop and do something completely different for "just because" reasons. On the other side, Odyssey throws all kinds of meta shit at you. You can spend all your time on the water, upgrading your ship, and killing pirates, or military ships, or be a pirate yourself. Also each region is ruled by one of the two factions (Sparta and Athens) and you can weaken it down by invading forts and what not, then join in on a large scale battle for one side or the other. I'm finally used to it now, but at first it was really overwhelming.
Since I'm not an old school AC guy, I feel both games really suffer from the jarring modern day shit. I get it, and I know the purpose it served in the older games, since it was really what the overall plot was about, but in these games it just feels forced and tacked on. I've heard that the ending to Odyssey sheds enough light to make it all make sense, so I'm really hoping that is the case. Thankfully so far in Odyssey they have been really brief if you wanted them to be. Some of the sequences in Origins took for fucking ever and were terribly timed.