I just picked up Dark Souls yesterday. I haven't played Demon's Souls, since it's PS3 exclusive and I'm on an Xbox.
I can say, however, that Dark Souls is thoroughly raping my ass, though I'm not sure if that's a good thing yet.
Not a fan of the controls. Shoulder bumpers as the main attack and defend buttons are very odd--I've never played another game that had a similar control scheme. I've only played for about an hour though, so I'm hoping it grows on me and that my hands get used to the strange control scheme.