If this were a 5 rounder, I think Bisping would stand a much better chance. I think a lot of people are underestimating Bisping working from the outside while overestimating Sonnen's mma wrestling skills.
That being said, I think this will be a UD, probably a 29-28 for Sonnen. Round 1, Bisping will be able to work from a distance and will be prepared for Sonnen's aggressiveness. Rounds 2 + 3 to Sonnen.