I agree. It's a little bit of both. Right now it would be Kelly due to PxP, her baby, and American Idol ending. Soon it'll be Pink taking over, then Kelly afterward with her new album.
I guess Kelly, she has a safer image and a more likeable personality along with appealing songs.
P!nk, on the other hand, is used as well but her image and songs are more edgy and "in your face".