The Queen was born to be under the spotlight.
Britney was unquestionably the driving force behind the return of teen pop in the '90s. Her career was high profile from the very beginning and she would have been that force even if she came out a year before or later, it wouldn't matter.
Christina was successful, even though she pretty much peaked with her debut, but Britney wasn't just a star - she was an international sex symbol and a phenomenon, the defining figure of American pop culture. Her innocence appealed to her female audience and the titillating and still unformed sexiness enticed so many male fans. In addition, she was a controversial media fixture and among female singers of the era, her celebrity power could not be rivaled.
Christina was, is and will always be incomparable in every way and far bellow Britney's level.