While Christina is the stronger singing talent, I think Britney is sexier. She definitely has more curves and bigger boobies, plus a really cute smile. Christina can be ravishing when made up right (like in the Ricky Martin video and her own "Come On Over" video), but I have also seen her look dreadful, with that corn-rows hairstyle and too much makeup. She is also way too skinny for me--sometimes she looks like a blonde Blow-Pop! Plus, I get the feeling Christina would be a stuck-up prima-donna bitch, while Britney would be nicer and more down-to-earth.
But I'd rather have Gwen Stefani or Shirley Manson than either of them!