The games are totally different in approach, Dont Starve is a harsh survivial game but on a 2D plane, with more on gathering resources, Valheim is a 3D game with more emphasis on staking your place in the world.
Both can be enjoyed solo with no issues, both have MP options for those who want it.
Valheim does the more enjoyable combat, though it is still basic but it feels better.

I think both are worth your consideration, if I was to be pushed on one or the other, and I have no experience in either game, I would say Dont Starve for now, its a done project but Valheim has a lot to keep you going despite not being completed... I dont play a game for 200+ hours for nothing.

I say if you can afford it, try both but if its a one or the other... Dont Starve is done, but has DLC you may want to buy that increases the costs, or just get Valheim which at the moment is pay now and get virtually all of the game (Odin pre-order is the exception).