Demon Souls is harder than Dark Souls. Citing from a friend who played extensively on both. As a beginner the game will be harder than normal but as you become more used to the games and knowing the levels you'll just start steam rolling right through.

That being said the difficulty mostly comes through the lack of knowledge of an area. Playing through an area or knowing what's ahead greatly cuts down on the difficulty.

There are no choices of difficulty. It stays static and increases through New Game + (people often abbreviate to NG+). In NG+ you keep all your stats and items you built in the first play through and play the same exact game with slightly harder enemies.

Also PvP in Dark Souls is atrocious. Everyone builds the same critical build and wears Dark Grain Wood Ring to reduce armor encumbrance. Or they build +10 Lightning Zweihander or Washing Pole. Also the latency is terrible, most of the time you'll die without seeing the other person even move, it's called lag stabbing. I suggest playing offline until you reach a boss that you would like to Co-op with before you are comfortable with invasions. People will invade over and over and over and over and over just to be a dick. On a related note you will only be invaded if alive and not a hollow. Some levels come with NPC invasions that reward handsomely as long as you were alive to get the invasion.

Also learning to parry is an essential tool as the final boss is an easily parry gibbed nut sack. Learning his parry timers are a little tricky at first though but once you get it, he's a joke. Also you never get to craft his sword which is lame.

Also amazing Pro-tip if you do get the game, do not level stats past 50 as the stats suffer astonishing diminishing returns after 50.

Dark Souls is a fun game to play after you understand how everything works. For newcomers it is often frustrating and not worth the effort but you will have fun with it if you like initial challenges and enemies that you don't just plow through without thought. I advise you to get the game and a large HD Television, the text is way too small.