I played Dark Souls first, so that was a challenge to begin with.
I personally think Demon's Souls has a much tougher level design (enemies, layout, traps). However, Dark Souls had significantly harder bosses.
I mean c'mon, the only challenging bosses in Demon's Souls were Flamelurker, False King Allant, and Maneaters. Then there was Old King Dorann which was optional.
I mean most bosses in Dark Souls posed a much more significant challenge other than pinwheel and moonlight butterfly.
When I first played Demon's Souls, I literally beat these bosses on my first try - Vanguard, Phalanx, Tower Knight, Dragon God, the Doll, Old Monk, Adjudicator, Storm King, LeechMonger, Dirty Colossus, Astraea and Garl, and Penetrator. I never did any researching beforehand. I think they balanced out the bosses and levels better on Dark Souls. In Demon's Souls you would proceed through a mind numbingly controller throwing level only to fight a boss that went down like a b****.
I seriously only died on the False King Allant (medium), Armored Spider (few), Flamelurker (a lot), Maneaters (a lot), and Old Hero (few).
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Both are easy.. I am a cheap player when playing pve.. enemy placement in demons is a little annoying but when you know where they are you can fly thru levels along with the shortcuts.. bosses were all pretty easy since you could use cheap tactics against all of them.. being invaded in demons is where the rush was at.. dark souls has harder enemies but bosses are all easy as well.. shield up dont rush every enemy has a flaw to be exploited..
dark souls is harder simply cuz of the ledges and falling off because of how slippery your character plays.
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Probably whatever you play first.
I played Dark Souls and that was really difficult the first time through, sadly I used a long sword alot of the way lol didnt know how to upgarde or nothing, so I was doing really bad damage, didnt know about the dragon sword you can acquire really early...it was a nightmare lol
Demon souls however, I played after Dark cause I loved it so much, needless to say the mechanics where basically the same so I pretty much skimmed through this game with little to no difficulty, except the boss in Latria 3, goodlord he was difficult, but bosses people had trouble on like Penetrator or Old king was very easy to me
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Dark Souls has more hard bosses. Demon's Souls is faster-paced so the non-boss gameplay is harder as a result (along with other things mentioned like cheap positioning although the Silver Knights on your way to Solaire's bonfire in Anor Londo takes the cake for cheapest positioning in either game).
That said Demon's Souls will flat out be harder no matter what if you play on Pure Black world tendency. It's the equivalent of having a constant Gravelord curse on your world except ALL the enemies do more damage, not just the Black Phantoms.
It's difficult to say for me. Because of all the mechanics and skills that carried over from Demons' Souls to Dark Souls, I started out at a much higher knowledge level and already had the right instincts in Dark Souls.