You know, I don't think they really had the effectiveness of types in mind when they came up with the story that Cresselia hunts Darkrai XD; I'm pretty sure it's more of a symbolic meaning, the moon chasing away the darkness, etc...
But if the two had I fight, I'm not sure who'd win :[ I guess Cresselia, just because in the one battle I used it in I had more luck with it then the one battle I used Darkrai in XD