I haven't played Ark(Seen some gameplay though), but I've got around 250 hours in Rust, and I am safe to say it is such a fun game to play with friends, of course it's hard to start like with every survival game, but find a nice server and go and play with some friends and you'll have a great time. There's a decent variety of guns, and other stuff to craft. raiding with stuff like explosives is such a blast(Haha so punny) and in general I'd probably say I'd have more fun playing Rust. Ark however has dinosaurs which is bloody awesome, but I think it might be a bit too early in development to buy(?) it's still quite laggy if you haven't got a good computer. All in all, my preference would probably go to Rust, just cause I know how much fun you can have with it, really growing and modernising alot is great, and I think(From the gameplay I've seen is less 'Primitive' than Ark.