Both are enjoyable in their own ways, but also function a bit different. In Valheim you'll likely being leveling up RPG style and building Minecraft style. You can't do either in Astroneer, as you have pre-defined structures that you just plop down. It's like a Factorio Super Lite. Though both are just as relaxing, and Astroneer doesn't really have bosses, or any enemies. It's more about exploring and the journey. Worst thing you'll find is stinky plants. Still fun, though. I'd probably say Valheim, but Astroneer is worth at least one playthrough.