Lesnar will win the first match, but they'll go at it at least one more time with Punk somehow pulling out a victory the second go around. Not that I expect Heyman (or Axel) NOT to interfere in the match at some point, but when it comes down to the end of it, I don't see why Brock can't pick up a clean victory. Lesnar is one of the most credible heels ever as far as pulling off clean victories - The Rock, Hulk Hogan (first man out of all those monsters over the years that ever successfully dropped Hogan into unconsciousness with a bear hug), Undertaker (in a HIAC match), Triple H at last year's Summerslam, the way he destroyed Cena last year and even had him pinned at one point had it not been for the referee being knocked out, etc.
I'd shake my head if they had Brock go over Punk dirty. Punk losing the first match and then coming back for more until he puts Lesnar down plays well into his role in the angle as the underdog.