I absolutely love 7 days to die. It's probably best played in coop with a few mates, but I also enjoy playing solo. The PVE is the best I have encountered in any of the survival types games I have played and has a really in depth leveling, crafting and building system. I haven't tried playing PVP, but I'd imagine it wouldn't be the best choice for that. For £5 its a steal, considering i have like 500 hours in it and still not bored as the devs are still actively updating the game and each update adds a ton of new content.
The Long Dark does also look like a great game. You won't go far wrong with either. I'm going to pick up the Long Dark myself I think.