This is a hard argument to make because although at some points Britney has outsold Christina and vice versa, they are both insanely successful. But remember, they're different people, they have different strengths and different weaknesses.
Christina is by far a much talented singer, with a wider range and powerful voice. Britney Spears may possibly be a better marketer, in that she markets what she has very well. (Not to say she can't sing, but her vocals are very lacking) Christina is known to stay true to herself, "Stripped" and "Back to Basics" are prime examples of that, while Britney does the same with "In the Zone", only not as strongly. Britney has had more successful tours, however, I don't think Britney is the better performer necessarily, if you watch the Grammys from 2007 Christina owns the stage.
They both have their own styles and are great. As a Christina fan, I may be biased, but I do believe she's more talented, however I don't think she is as smart about marketing herself.