Since 1999, when Britney Spears and Christina Aguilera both entered the music scene, fans have been endlessly debating over who is the better pop star. Not an easy question to answer, since the two have always had very different styles. But a leaked demo track finally gives us a chance to hear the two on the same song, perhaps giving us an answer to the age-old question, which pop princess did it better?In 2004, Nelly released a song called "Tilt Ya Head Back," which featured vocals from Aguilera. But, according to an interview the track's producer Dorian Moore did with the St. Louis Post-Dispatch that same year, Spears was originally offered the cameo. However, her record label Jive deemed it too "urban."The rumor was Spears passed on the song, which was then offered to Janet Jackson and Aguilera, who eventually landed the spot.Now, there's confirmation that Spears did actually record the song, all those years ago. A Singapore radio station played a 30-second snippet of the previously unreleased demo, which doesn't sound too different from Nelly's Curtis Mayfield-sample final version.