gaga sold almost the same amount of albums
won more grammys as a solo artist than Rihanna
tour wise gaga
sales wise gaga
awards wise gaga
impact wise gaga
singles wise rihanna >
Gaga's peak is certainly bigger than Rihanna has ever been or is today, and Gaga's album sales are more impressive considering she has fewer releases and in a shorter space of time. Rihanna's single success remains mammoth though, and although she hasn't matched Gaga's touring receipts she's a worthy contemporary.
Rihanna has had the bigger career. More memorable hits, more overall successful years and her prime (so far) lasted 7 years which is 4 years more than Gaga's.
Gaga has had more impact than Rihanna on pop culture though.