The past few years have just been ROUGH on Britney. Even when she's dolled up to perfection, she still gives me 38-year-old aunt who smokes-teas. There's just something about her face (especially her eyes) that just looks worn out. Comparing her 2003-self to now is startling.
Christina on the other hand (if we look past the weight/make-up issues) looks about the same as she did 10 years ago... Her face still looks smooth and youthful.
Here is a non-biased 2003 vs. 2012 comparison....