The song suited better Rihanna honestly , but I assume as a songwriter / producer Shakira is she is the brain and responsible of the song ..... So both were important I guess.
First: This is a song more friendly for Rihanna's vocals.
Second: Nevertheless, this forum has a LOT of Rihanna stans/fans and lacks Shakira's.
Conclusion: Eventhough Rihanna's vocals are more fitting of this song, it doesn't mean she owns the song, the thing is, this duet, in ATRL, was always about Rihanna, so even if she sucked, you all would say the same thing.
Both of them sound really great, this song is pure slayage. What's the idea to say Rihanna owned Shakira? Shakira will perform this better than Rihanna in live presentations.