Spoiler
I'll try in this comparison to take each element of the game separately and will begin with
1. The Map.
Now, by any standards the AC Odyssey map is impressive. I've covered what, a third to a half of it and I love the use of the ship to move around it; I love the mountainous terrain and the way that you can set your horse following a road and track him from the air as he carries you into new territory, racking up XP simply by passing through regions and past locations. In terms of its variety however, to date I think the Skyrim map has it beat. I keep seeing these snow capped peaks and I want to get into them just to get some variety from the sun-bleached Greek terrain, but to date have been unable to reach them; I don't know - perhaps you do get the chance later on or in parts of the map that I haven't reached, but I'm beginning to find the countryside a bit 'samey' and am looking for some variety.
In terms of the actual locations, I love them: the degree of accuracy maintained in the historical locations that really existed - and still do in their archeological state of today - is stunning! I love the historical notes provided on each site (for those of a historical bent) and to be able to wander them almost as the real people of the day would have been able to is simply wonderful - worth the purchase price of the game alone. But this is a comparison, and as a purist looking from the terrain alone, I haven't yet seen anything that matches the variety of the cities of Skyrim, the different types of terrain (mountains, plains, Forrest's, etc) and of course, the crypts. In respect of the latter, in Odyssey to date I've been into a dozen or so cave systems, dived into a few underwater sites, but nothing that comes close to the Dwarven ruins of Skyrim, the underground crypts of the falmar and draugr, neither in complexity or scale. Again, perhaps I simply haven't encountered them in this map yet - but if they were there, I'd have expected to have come across them somewhere by this point.
Next lets look at
2. The Quests.
Now this is I'm afraid, where I'm beginning to have serious problems. In other huge role-playing games (and not just Skyrim - think Fallout 4, the Witcher Wild Hunt, KCD etc) once the player is set loose into the world, before long they are directed into the main quest (all good here with AC Odyssey) but also toward secondary quest groups that have as much, if not more, interest as the main quest itself. In Skyrim, before long you find yourself in Riften, getting hooked up with the Thieves Guild, and entering onto a quest-tree that draws you in with an ongoing story of depth and interest, and in which you become invested as you proceed through it. In the aforesaid Guild, you do a few initiatory tasks, discover that all is not well in the Guild, and then work your way through an extended story to the resolution of the problem. It's long, it draws you in - and the game has dozens of them!
In AC Odyssey, I've searched in vain for this kind of additional content, but to date have found nothing but simple, one of tasks and contracts (kill this Athenian leader, sink those five ships) which you carry out and then scoop your XP up for. The site objectives are similarly uniform; kill the Captain, loot the treasure, find the tablet. Where are the engaging side quest-trees of the type I mention above in Skyrim. In desperation I went online to see if I could get a clue on this and found a site where a guy was singing the praises of the side quest "Lust for Life", as one of the best in the game. I've done it; you collect a bear scrotum and a deer horn and take them back to an old woman who you get to shag or take payment from. That's it! It took me ten minutes to complete with nothing of interest to me personally - and this is the guy's idea of the best side-quest! If this is seriously what all of the side stuff in AC Odyssey is like, I need to plough into the main quest real fast and hope that the immersion I crave from a game like this can be found in there to make up for the truly uninspired nature of the side missions. Either that or just hope desperately that one of you guys can say "No Peter - you simply haven't got far enough into the game to reach the interesting side stuff". Perhaps that is it; perhaps the game is simply so huge that I haven't reached the "Thieves Guild", the "Assassin's Guild" the "Labarynthium and BlackReach" of it yet. Or is it simply that the main quest storyline is so staggeringly long and good that it compensates for the repetative and packing-filler nature of the rest of the content.
Don't get me wrong however. I'm massively, massively impressed with some aspects of this game. I love the map, I love the ship and the horse riding, I love the bird and the historical locations, the attention to detail and the ability to walk the streets of ancient Greece, the Parthenon and the Oracle of Delphi. But, hell, I need something to do in it and I'm sorry, getting my leg over an eighty year old woman who's just eaten a plate of oysters isn't going to cut it! Please, someone tell me I've got it wrong; that the main quest will transport me to another place or that the side-quest quality will go stratospheric very shortly - because I'd hate to loose all of this beautiful historic accuracy, this rich and detailed landscape, on the basis of a failure of simply providing a good story.
3. The gameplay.
To date I'd say that Odyssey has Skyrim beat. Even in the remastered patched version of the ps4, Skyrim is full of bugs, glitches and the like, that while they rarely impede the game playing itself, can be irritating and annoying. I haven't yet come across any real bugs in Odyssey and its only stopped once with an error message (in Skyrim it was happening almost daily in the en). The save system in both games is I'd say comparable; outside of combat situations you can save pretty much where you like and similarly with fast-travel, both games are comparable.
I myself prefer the first person perspective of Skyrim to the third of Odyssey, but I recognize that the emphasis on platforming (climbing to me) of the AC games is not suited to this - and I do love this part of the game. That climb to the 'synchronize' point followed by the dive into the pile of leaves gets me every time. How can you not love that!
All in all then (and I assume that you've guessed this by now) I'd have to, in any vote on the subject, come down massively on the side of Skyrim - but that's just me.
Tell me what you think?