BS. MUCH bigger in terms of streaming. It is @ 7.5M despite being released in November 2014 (there's a 1 year 10 months gap between Stay and BS just fyi) and it helped 1989 sell tons of copies.
I'm voting for BS even though it sold less not only because it's streams make up for the 9m to 7.5m difference but also because its mother album is so much bigger.
So since Stay outsold it, you are using the album sales excuse to prove you're right? Like it had something to do with the songs success... using this logic even Style was bigger than Stay.