Demon's Souls will always be the better game IMO. The gameplay was a lot smoother in Demon's Souls, not that it was bad in Dark Souls, but Demon's Souls just felt better to me. I also really liked the hub system and the world- and character tendency systems. Dark Souls felt like a more linear game to me in the sense that I felt I should approach areas in a specific order if I wanted to make any progress.
I preferred the boss battles in Demon's as well. I will never forget the Flamelurker and how much trouble it gave me. Some of the other bosses and NPC's are also a lot more memorable than any in Dark Souls e.g. Penetrator, Tower Knight, False King, Patches, Yurt the Silent Chief, etc. I can't think of one NPC that stood out for me in Dark Souls, granted I haven't finished it yet. I have finished Demon's Souls 3 times though (and lost all of my save data

).
Will definitely still get Dark Souls 2 as well, but I will be waiting for the PC version.