i'd say they are about equal overall.
demon's souls is probably harder overall now that dark souls was patched to nerf a lot of things. i really liked the persistent overworld thing they did with dark souls but towards the end of the game (where shortcuts were less frequent) it was kind of annoying to repeat areas again and again. demon's souls you could warp into any area you wanted which was kind of nice. and i preferred the body/soul form in demon's souls over hallowing in dark souls ... it just felt like there was more to distinguish each form and you had more motivation to go into one or the other at times. also, the few cuts scenes there are in these games were done better in demon's souls. i will never tire of watching the fat official get impaled.
okay, so maybe i have a slight preference for demon's souls. still pretty equivalent gaming experiences.
