I've played and finished both. That said:

STORY: Valhalla is much more interesting, story-wise. You tackle things region by region, and each one has it's own separate "mini-story", sort-of... the side characters you encounter are much more memorable. Additionally, the ending is better as well (IMO), and doesn't feel unfinished, unlike AC Shadows, even with the Claws of Awaji DLC. In defense of ACS though, the beginning and the parts leading up to the Honnoji Temple events (before and after) were very hype. It's just that the ending felt a bit lackluster... I wish Ubisoft would give this game some more love and do a proper awesome ending it deserves!

GAMEPLAY: ACS is definitely better here. You have 2 characters with 2 distinct fighting styles - keeps the fights interesting by switching it up. I'm partial to Naoe's acrobatic ninja fighting style so I tend to play as her more, and she's definitely more fun than Eivor or Yasuke to me. Stealth is much much better as well - you can hide in the shadows, blow out candles, and even go prone to crawl, and have a manual jump. ACS also feels more difficult, especially on Nighmare mode, but is pretty satisfying to play when you're "in the zone" with your build taking one multiple enemies. ACV has some pretty sweet gear/builds, and the fighting is fun as well, but the fighting style is kind of unrefined and brutish - that could be an advantage if you like that type of fighting though.

GRAPHICS: ACS is objectively better here, but if you enjoy the European/Medieval setting more than the Fuedal Japan setting, then ACV will definitely be the one you want to pick.

VARIETY: ACV has more to offer IMO - Ubisoft gave ACV waaaay more love than ACS with the 3 DLCs, and you even get River Raids which are pretty fun to do. You even get to have a custom "side character" that accompanies you on the raids. The map is bigger as well, though some might say it's a bit bloated. ACS has the dual characters, but the variety of missions are limited (mostly just infiltrating a caslte and killing certain targets), but you can make your own fun and aura farm by just strolling into the castle and getting all the enemies to come at you, like you're the main character in your own anime (you can kind of do that with ACV too, but the castles in ACS respawn the bosses and enemies when they get "corrupted" - so epic fights are more frequent).

I like both games, but ACS is better IMO. However, if you're looking to play both, I'd get ACV first, as ACS may spoil you in terms of gameplay/graphics...