"Pretty" is pretty general. Neither are pretty.
Beyoncé is beautiful where-as RiRi is younger and got a hot vibe going off her (she always being sexy in her songs/videos). Needless to say, that doesn't mean RiRi has never looked beautiful and Bey never looked hot. It's just their general image as I see them.
They're also off different race/age so a comparison is stupid
I'm gay so I'm not the best judge for beauty in women but I've always thought Rihanna had an attractive face, while Beyonce's face has never done anything for me.