I finished Demon's Souls recently and I've come to the conclusion that Dark Souls beats it in almost every category except level design and arguably level variety. There are many little things that are done right in Dark Souls that they stack up and overwhelm Demon's Souls, from things like the simplified upgrading system, interconnected areas minimizing loading, removal of carry weight and the stupidity of not being able to carry something you pick up, letting it drop to the floor and it disappearing forever, 50% of the NPC interactions tied to an obscure world tendency that requires a lot of counter-intuitive gaming to work, limiting healing items from abundant grasses to Estus Flasks. Dark Souls has more magic, more weapons and weapon variety, covenants, and NPC quests that you can take without being hidden behind some crazy mechanic. You can just warp to another place without having to go back to the Nexus, wait for loading, go into another world, wait for loading, spawn at destination. The majority of unique weapons are useless (i.e. not viable, not that it's only used in power builds) because Colourless Demon's Souls are extremely limited if you're not willing to tank your world tendency. The addition of poise increases the value of heavy armour as a tradeoff for mobility. The one thing I wish was carried over is trading boss souls for spells, which you can only do with one spell in Dark Souls.
So yeah I think Dark Souls is superior to Demon's Souls in almost every category, not to say I didn't enjoy Demon's Souls but I like Dark Souls a whole lot more.