the reach
Rih is more respected from the Grammy's to the GP, that's it.
And the Grammys she has won are hers too, i mean who cares if it's a duet or not? (Crazy In Love is a duet but no one denies that the Grammy winner is Beyoncé), who cares if it's for a dance track (Toxic won in that category too but it's still counted as the only Grammy Britney has won)
Said that, Grammy's doesn't make an artist greater than another... there are too many things to count, well in my opinion Rihanna > Katy, but who cares?