Both are nice to look at.
Being serious though I think that I like more Rihanna songs more as I like songs like California King Bed, Umbrella, Diamonds, Sledgehammer, and We Found Love.
Where as with Beyonce I do like Halo, Crazy In Love, and If I Were A Boy, but then I take or leave alot of her other songs.
Vocally though I do like Beyonce's voice alot more.