Katy will matter more than Avril when it is all said and done. Avril is responsible for making people wear ratty multicolored extensions in their hair, and that's it. Katy is known as ONE of the leading pop girls of her time. Avril was an afterthought even back in her peak. There was Christina, P!nk, and even Belly.
It's not just about the numbers. In 2014 if you say "Avril Lavigne" what will people think of? It's that first mental image that an artist's name conjures that says everything about their importance/legacy.