This is a tough choice because both of them are beautiful. In the end though, I voted for Keira because Natalie strikes me as the type of girl who wouldn't want to have sex all that often. I could be wrong, but she seems like she would feel like she is above it for some reason.
On the other hand, Keira seems like she would be totally into a good night of sex. She just seems like she would be a lot more fun.
Keira also has a tighter body too. Not that Natalie's is bad at all.