It’s hard to tell at this stage as they both have around the same number of prime years. Britney Spears album sales went downhill the second part of her career while Taylor has remained steady ever since she debuted over a decade ago.
I am not sure who has sold more albums but I guess it’s Britney given her explosive album sales for her first theee albums alone. However this is not a metric that can be looked at alone because album used to sell quardruple the amount of what they sell today on average. Britney album sales performed better than her peers but not by a margin whereas TF album sales numbers are unheard of in this day and age. Again, relativity is key.
Then again, Britney’s has had more influence on the music industry as it was transitioning from traditional to digital. Britney gave us more iconic performances and moments (both good and bad). She was also a bigger phenomenon and a bigger star overall. It would really help if there are established metrics that can helps us fairly assess “success”.