Dark Souls is a better concentrated experience, there's a lot more to that game in terms of combat mechanics and the multiplayer is a unique and well implemented experience, so I'd say Dark Souls is the better game to 'play'.
Skyrim is the better roleplaying game, i.e. creating your own character, playing them like a person, creating their motivations and choosing their role in the world etc, it's a lot more diluted than Dark Souls in terms of combat, but it makes up for it with how immersive the world is.
That's not to say that Dark Souls doesn't have an interesting world, it does, it has great atmosphere. But after playing the game for so many hours, the world is actually tiny compared to Skyrim.
It's funny, I remember first playing Dark Souls and feeling so overwhelmed by the world, it seemed huge, but once you've been everywhere, you realise just how well FromSoftware managed to make so little seem like so much.
