I don't know but isn't the close album sales a given point for Britney since she is still competing even after her peak? Beyonce may have had better album sales but the difference is really not big. It's an indication that Britney still has what it takes to compete with Beyonce and the other popstars.
She managed to get 3 top 10 singles last era, a feat Beyonce didn't have with 4. All those top 10 hits went platinum. She had a successful tour. She raked up more money than Beyonce last year according to Forbes (
http://www.forbes.com/celebrities/) and right now she's being paid 15 Million US Dollars to judge the X Factor.
Verdict:
I'm still not keen about Beyonce being superior than Britney in any aspect except for bigger album sales as a whole but as of right now Britney Spears is by far superior than Beyonce.
That's all
