I like Brock, but I'm going with Frank on the big stage.
He has more experience, better submission skills, and, well, he already beat Brock once.
Brock's experience in the UFC really hasn't been against fighters at their best - aside from the Mir fight which he lost - with Couture coming off over a year's hiatus (and being in his mid-forties) and Crazy Horse (was it Crazy Horse he fought?) being a more or less under card type of fighter and little more than a stepping stone provided by White for his "next big thing".
Anyway, my money's on a first round submission victory for Mir after Brock - once again - forces the fight and ends up making a critical mistake - like leaving a leg unguarded.:2 cents: