Does Gianna fall outside the voting window of what constitutes a 'Newcomer'?
I'm just surprised that she doesn't get more award-love seeing as she is to all intents and purposes the MVP of gonzo porn at the minute.
As to the question in hand, it's a tough one. My own preference would be for Mia because she has such ridiculously perky tits (a trait shared with sis Ava), but in terms of who has made the bigger impact on the industry, then I'd say it's a straight duke-out between Naomi and Sasha Grey, with Naomi getting the nod simply because she's been around a little longer.
Still, this is pre-meth Naomi we would be talking about here, not the current skeletor-like incarnation, and even then, I'd still say looking at her ass is preferable to looking at her face.

But hey...most of these awards are determined by advertisers anyway, y'know?