As much as I respect and enjoy listening to both artists, both are kinda generic pop music. To me, the only one that really stands out for uniqueness is Lady Gaga. Katy Perry has perhaps set trends and is the woman every teenage girl wants to be, but didnt bring as much innovation to the table as Gaga did. I think Gaga's career has been slowing down in comparison to Katy Perry at the moment; but as far as those two album goes, Lady Gaga's definitely had many songs that hit the top lists so hard, I don't have a doubt they will be regarded as defining songs for its decade.