to decide between Lee and Clementine. Lee is not bitten yet.
That's the scenario I imagine: Lee is grabbed by a walker from behind, another walker falls on Clem like the one did on Duck before biting him, his had crushes on the street, he raises his head, staring at her throat, then she quickly gets out her glock and pulls the trigger, it's jammed, he drops on her, so she sticks the gun's barrel in his mouth and tries to keep him off, she can't do it too long...
Lee is holding "his" walker off with his arms and hands meanwhile.
Now you (Lee) can get your gun out and whether quickly push back "your" walker and shoot him, as a reaction to that Clem cringes, so she can't hold up her guard, and the walker gets her, since she still is 9 and that gunshot could scare her, or you push away "your" walker, then turn around let your own guard down, and shoot "Clem's" walker, so she is save, you can't be quick enough to kill "your" walker too though.
you've got 3 seconds, no slow-mo' nothing, just your instinct.
I'd save her with no hesitation, what about you?