I like Dark and Demon's about equally. I think for the most part Demon's has the better atmosphere (Dark's mainly atmospheric in New Londo), but Dark has better roleplaying opportunities.
Yeah I agree, Demon's Souls is a great experience while Dark Souls is good game. The only that FromSoftware needs to do in the next title is combining this two things and they have one hell of game to offer, and if they can come up with something new then that would be even better.
The only question I have is if this next game ever happens there's a possibility to be released on PC or not. I mean now with Dark Souls coming to PC is pretty obvious wondering about that, not to mention that there has been quite a few people getting bored of consoles and going to PC in the last few years.