I'm going with the simplest answer; this is Cass. We'll then see more "I am Batgirl" images of other female characters, just like DC did with Battle for the Cowl.
Also, I've realized that the best possible outcome (for me at least) would be a book starring Cass AND Steph. I honestly don't care who's in what costume, I just want a series with the both of them. Some of the best moments of Cass's book were the interactions between the two.