Supergirl stands out the most to me because she had her own live-action, big budget movie, even if that was awhile ago. That might be weak reasoning, but it gives her a sense of being more of a stand-alone character in my mind. I'd consider playing Supergirl. Power Girl is like a hotter version of Supergirl with a better costume and huge tracts of land. As a player, I really don't mind if Supergirl and Power Girl are just skin changes for Superman. As long as there is sufficient variety with other characters, it does not bother me if some are like that. If NRS came out with more MK9 costumes, I would buy them. I like that MK9 has some subtle differences in costumes too, like Cyrax/Cyblax, but even if there isn't, I like costumes. I'm one of those gamers that spends hours in a customizer.
I never cared for Batgirl or even Robin for that matter, with the exception of Carrie Kelley from Dark Knight Returns (haven't seen the movie). If the reason for not having Poison Ivy and other characters from Batman is to not have Injustice be too Batman heavy, I'd prefer not to squander that precious Batman quota on Batgirl. Sorry.
For what its worth, I'd much rather have both Supergirl and Poison Ivy than Batgirl. Can anyone else see Poison Ivy doing a move like Sonya's Kiss?