This is easy and obvious!
Avril had no excuse, birtney spears is older than her, has released more albums than her and has a longer career than her, avril was the one in debt to sell more.
The reality is pretty basic
Femme fatale outsold GL
Femme fatale received better reviews
Femme fatale won more awards and nominations
WTH only outsold HIAM in US and Japan, in the rest of the world HIAM is a bigger
TTWE literally killed Smile
IWG destroyed WYWH
Femme fatale tour was like the double or triple of succesfull avril's tour
Brit had so much better year end charts positions
And actually even in Canada avril's own country Femme fatale is certified platinum and GL is uncertified
So there's no doubt Britney had a better year