Those are two completely different genres and games.
If you haven't played Demon Souls yet there really is no need to get Dark Souls yet. Dark Souls is pretty much a sequel but costs right now 60€s while you can get demon souls for around 20-25€s right now and the game definately is one of the most unique and best PS 3 exclusives.
HOWEVER you should be used to play REALLY hard games and I hope you don't rage. Demon Souls is insanely hard and Dark Souls even more so.
Playing Cod games on Vet are literally a piece of cake compared to those games.
No real opinion about Rage so far. All I heared and read is that it's supposed to be a mix between Fallout and Borderlands but with a pretty boring story and a lack of innovative gameplay.
Personally I'd wait for the first reviews when it comes to Rage before deciding.