I think ARTPOP will sell more in the long-run. Worst case scenario for Lady G is her songs/theatrics are too polarizing & out there, than I would't be surprised if Kathyrn gets ahead. Never underestimate the competition.
First Week - ARTPOP
Long Run - Prism (it's getting another huge campaign like TD)
But Teenage Dream literally had the biggest promotional campaign ever, including a movie and re-release, yet Born This Way still outsold it. So why would Katy win this time?