Natalie is sexier: her face is far, far more beautiful than Britney's. Natalie's is a world class, timelessly beautiful face, whereas Britney has a reasonably pretty face.
Natalie is also infinitely more intelligent than Britney. Fucking stupid, but attractive girls like Britney is fun, but sex with someone beautiful and really clever is vastly more satisfying and exciting.
Britney has a better body, that much is true, but it's no contest here.