Britney. I like Avril too, but she was too slow to release singles and she didn't even release a fourth single like she has always done since her debut. Britney had awesome singles and videos, HIAM and TTWE were SMASHES and the perfect kick off for the FF era, IWG was an amazing followup to those smashes and consolidated the new era and "Criminal" even though underperformed (to not say "flopped") was the perfect way to end the era with an EPIC video beating HIAM, TTWE and IWG together. The tour was also amazing, still not close to the amazingness of The Onyx Hotel Tour but an improvement over The Circus Tour (she danced more and lip synced less). Overall, Britney shined more than Avril in 2011, but if Avril couldn't be bigger than Britney it's her fault for her slowness to release singles which is a KEY part of the promoting process.
I would say Britney did better by a hair when just looking at the numbers, but in terms of pre-release expectations Avril did better
Britney went on to have one of the biggest tours of the year and had several hit singles and sold more albums. How exactly did Avril do better? Nicole stop.
.