Both, Katy and Taylor are arguably the biggest pop girls on earth right now. One released an album two years ago and the other one a year ago. Both got #1s, broke records and got Grammy nominations. Both are also currently touring, but, which one has had the biggest era so far?
I mean you could debate on the singles front (but even that is close between Roar/Dark Horse & Shake It Off/Blank Space/Bad Blood), but 1989 has outsold PRISM at a rate of over 2.5 to 1