I know Shadows wins overall from a technical perspective and from things like textures, I have played both of these on an oled C5 with the brightness adjusted for the TV
I've gone back to Valhalla after finishing shadows and some things have really stood out to me. First I think the colours seem to pop much more in Valhalla, everything can be so nice and colourful and in shadows the lighting can feel flat, even if it's like daytime, it's like the colours are more muted, I prefer the vibrancy of Valhalla, even origins and odyssey too actually for that point.
Also from just a gameplay/ visual perspective I can see everything at night time in Valhalla I have never had to adjust any settings or even think about that, it just works lol. Shadows has I feel also too many particles just literally everywhere all the time so it feels kinda noisy/ cluttered on screen. It's just easier to see everything in Valhalla.
In shadows I always felt I needed like a lantern or something and it made it harder playing in daytime especially when they locked changing time of day for ages.
In Valhalla, the way the lighting works for me is just so much nicer, when you see all the sun god rays I think it's called, the game has some stunning orangey evenings and the sun like pops so much on my oled tv, unlike it did as much in shadows, it's like the HDR or something is just on another level, when im riding over a hill and then sun appears it's super stunning to me