Since I've only played Demon's Souls(still waiting for Dark Souls to arrive), I can't really weigh them against each other, but Demon's Souls is a solid game that's a lot of fun. It can be really frustrating at times, but every time you die you learn something from it.
Unless you dodged your way off a bridge... Damn you, Maneater!
I guess it all comes down to if you prefer sandbox-type games or linear ones. Dark Souls is, as far as I know, an open world game with much exploration. Demon's Souls levels are very linear, but you have lots of choices when it comes to picking which world and which step to go to next.
Started a magic user a few days ago, to prepare me for Dark Souls, and I'm surprised at how unbalanced it is compared to my old melee character. I kill mobs in two-three hits(from a pretty long range as well), so going through the levels are really easy. However, if I get hit twice I die... but you're not really supposed to get hit in these games anyway.
I enjoy melee a lot more and oh dear I'm going pretty off topic here, best to just post and leave.