Britney:
3° Britney Spears – ...Baby One More Time – 24.000.000 (1999)
3° Britney Spears – Oops!...I Did It Again – 20.500.000 (2000)
4° Britney Spears – Britney – 11.000.000 (2001)
10° Britney Spears – In the Zone – 8.000.000 (2003)
Taylor:
10° Taylor Swift – Taylor Swift – 5.700.000 (2006)
3° Taylor Swift – Fearless – 8.100.000 (2008)
5° Taylor Swift – Speak Now – 5.100.000 (2010)
1° Taylor Swift – Red – 5.100.000 (2012)
This is the easiest way to approach it -- how each compared to the rest of the albums released in each respective year (which subsequently accounts for market decline, digital albums coming into prominence etc). Both scored 4 top ten albums WW, Taylor is a shoe-in for a fifth with 1989, which would come close to Britney's fifth release (although that was a Greatest Hits
).
As for singles --
Career length (weeks between first and last global top 10 hit as main artist)
13. Britney Spears 765
40. Taylor Swift 292
Taylor still has global top tens left in her for sure (especially with her still-growing global presence) so I bet that number can increase in the next few years once her next album is out and will likely yield another global top ten.
Obviously, I understand that Taylor was a country act for the bulk of career leading up to this album, but this is the most objective way to view it imo.