I have both. Having experienced Don't Starve, I assume DST is the same or similar. So based on that, I would surmise both are good. So it comes down to these things:

Valheim:
- Still in Early Access
- Was posted as EA in Febuary 2021
- Is in full 3D, not optimised
- Enough bugs that will need to be hashed out over time
- Balancing issues that will be hashed out as small updates over time
- Still lots to do before it fills in their road map concepts
- A few empty biomes at this moment in time
- Not all bosses are in the game
- Not stable for everyone

DST:
- Left Early Access in April 2016, which is over 5 years ago
- Has a lot of content through many patches over the years
- Is in 2D-ish, fully optimised
- Most bugs has been hashed out
- Balanced quite a bit already
- Devs still update the game

Both games can play multiplayer. So it's really a matter of what you prefer in style and what exactly you prefer in content. DST has a lot of substance because it's a finished product released over 5 years ago. Valheim is still an infant, that has a lot of growing up to do. However, Valheim can still be pretty fun, when it works for you.

Objectively, I would suggest DST for substance. I would suggest Valheim, if you want ambient beauty and a visual sense of achievement. However, unlike DST, Valheim *might* give you headaches coinciding crashes, FPS drops, visual incompatibilities, Linux-based server updates, etc.

Mind you, I've clocked about 370 hours with Valheim. Where as I have clocked only 5.9 hours with Don't Starve and haven't touched DST yet.