"Britney was massive during her peak!"
I mean, the question was on the first decade as a whole and Britney's two year peak isn't nearly enough to upset Taylor's amazing consistency and longevity.
But even if we were to only look at JUST the peak, Taylor is still bigger. Assuming Britney's peak is BOMT and Taylor's 1989.
1989 was THE bestselling album of 2014, both domestically and globally, while BOMT was outsold by backstreet boys, Santana and Creed.
At her peak, Taylor had
THE biggest tour of the year meanwhile Britney's tour grossed a measly $35 million.
At her peak, Taylor topped the Forbes earning $170 million, a number far beyond her competitors (and not to mention an additional $80 million from 1989's sales alone the year before, placing 8th). Britney failed to make top 10 in both 1999 or 2000.
It's like no contest. No amount of "but you didn't live through Britney's peak!!" is changing that. Actual stats >>> nostalgia