well, Grounded is released now and valheim is still in early access and missing features (like 3 more biomes have no content in them)

as to which one is better: its subjective, you might have a different opinion than i do

but i did play the free weekend they had and it was pretty fun (talking about grounded)
Both games are fun but again, if you are planning on buying one or the other, i suggest going for grounded as it is fully released now and i have no idea how long valheim will be in early access for. Its been a while already, but im sure it will get a full release eventually.

Plus grounded has a full storyline and valheim is a lot more sandboxy. so they are very different games tbh

Regardless of which one you choose im sure you will enjoy both of them, they are both really good
I know i enjoyed valheim more but I havent played it since i did all the boss fights, now im just waiting for the new biomes to get implemented.