Demon's Souls is mechanically harder because it's older, so the mechanics aren't as well developed as in Dark Souls. Case in point being parry windows being harder to connect with, and hitboxes being a bit too big/small in some cases.
Not only that, but the bosses, like False King Allant and and Flamelurker are hectic in a way which not many of the Dark Souls bosses can compete with. That said, bosses are more imbalanced, in that you can get their AI stuck behind a wall, and cheese them to oblivion. Even Old King Doran could have poison clouds tossed at his head to slowly tick away at his 10 billion health, whereas Dark Souls made bosses more air-tight in that regard (Bed of Chaos aside).
Dark Souls is more balanced, too, with all playstyles being equally (more or less) viable. In Demon's Souls, magic was OP and there's no question about that. Even a half-assed mage character could still wreck shit up by the end, whereas miracles were more of a support role, and a few were useless outside of PvP.
I've heard about people complaining about the first few bosses in Dark Souls, like Taurus and Capra. I never understood Taurus; use drop attack, hack at his knees, done. Capra is an ass, I'll agree, until you realise that he's entirely optional. In fact, some of the most annoying bits are completely skipable. Lower Undead Burg, the Depths, 2/3 of Blighttown are all optional. Demon's Souls doesn't do that; everything must be tackled and shortcuts are few and far between.
Demon's Souls is considerably shorter than Dark Souls, with playthroughs, for semi-experienced players, being 5-6 hours and that's with stupid deaths and grinding, while Dark Souls can double that easily. The difference is that the 6 hours must all be tackled, whereas the 12 can be trimmed down with clever shortcuts and speed building.
I find Demon's Souls harder, mostly because it's less forgiving. Fewer checkpoints and losing half your max life upon death is an arse, and that's without bringing up Character/World Tendency. By contrast, hollowing is completely immaterial.