Well, I'd say completing the core game of SMB, or even unlocking all dark worlds, is pretty much ont he same level as completing Dark Souls. If you're adding in extra stuff like Cotton Alley, then yeah, that's a completely different ball game. But the core games are about equal. If you see a massive difference either way, it simply means that your abilities at one of those genres are higher.
I also don't get the discussion that Dark Souls is not a hard game; and yes, there HAVE been people in this thread saying it isn't a hard game, no qualifiers, over and over. Without fail, all these people have mentioned there are lots of harder games and almost every single time, they fail to mention which. Leaving out indie games (which have brought us such gems as I Wanna Be The Guy), what retail games are harder than Dark Souls? Every one of us knows at least a couple hundred retail games this gen; if you can't name at least ten retail games harder than Dark Souls (core game, default difficulty), then you can't even argue that it's not in the top 5% regarding difficulty.