Who should
stay? Why, all of then, 'natch!

I voted for Kara as the REAL Supergirl, because she is. So where does that leave Linda? She can be Superwoman (she deserves it), and hopefully both of them will stick around.
As for Cir-El, it's really much too early to judge. We know next to nothing about her, her origins, and whether she's going to be a semi-permanent addition to the Superman family (as Kon-El and Steel were) or whether she's only a one-shot, existing for this single storyline.
And as much as I like Linda (and may or may not like Cir-El), I don't like characters with completely out to lunch origins. Can you summarize Linda's Supergirl origin in one sentence or less? I certainly can't. It's far too convoluted and nonsensical. For better or for worse, I regard a character's origin as an important component of that character. Compared to Linda's, even Kristin Wells' is a walk in the park.