Well, they're quite different games. I only got Astroneer because my older brother thought it was an interesting game, even though on the inside it was way out of my age group at the time. It is a good game, don't get me wrong, and I come back to it now and then to simply chill and enjoy the clean aesthetics, and developers provide occassional events to give players something to do after the core objective is complete, together with updates.

It's just a pretty "child-safe" game, so for someone past the school phase I'd rather recommend something more serious like Satisfactory or Stationeers, that are more challenging and intricate. If that's not even a consideration for you, and your co-op mates are positive about it, then there's nothing for me to say against Astroneer.

Valheim is more on the challenging side of things. It has very well-placed balancing both for solo and co-op, it is also well designed and looks great despite low-end models and textures, and it has features that appeal to builders, fighters and explorers alike. It is EA game, which means it can help some work, like zverozvero have said, but it is arguably a solid game already and an investment into a game that it eventually will become.

So in general, if you want something to chill, and the aethetic suits your taste, pick Astroneer. If you want something more challenging with a more long-term value, pick Valheim. The choice is ultimately yours.