Good question.
Drake could easily win with a great crossover single. Gaga is almost guaranteed to sell over 500K anyway. Drake can do the same. It's going to depend on the strength of their single and their promo.
If Gaga's ARTPOP app counts as a sale, Gaga >>>>>>>>. If not, Drake will come out on top.
Obviously, those are US sales.
WW, we know Gaga >>>>> Drake