BreatheHeavy
I hate to do the whole Britney vs. Christina thing, but...
LEFT: Britney during "Toxic" for the opening of her Onyx Hotel Tour in Maimi. - March 28, 2004.
RIGHT: Christina Aguilera during "Genie In A Bottle Remix" at the Video Music Awards. - September 07, 2008.
PS - she lip synched.