Just a bit of a hunch. Although Miller has a better record, he has really only beaten two opponents of quality in McGivern and Kimmons. Horwich has more losses but they are mostly to tougher guys than Miller has faced (except for the obvious fact that Horwich lost to McGivern, who Miller beat soundly).
And Horwich has some wins over very tough guys as well in Krzysztof Soszynski, Tiger White, Mike Pyle, Benji Radach, Jason MacDonald, and Josh Burkman. This gives him a big edge in experience.
None of this actually is a convincing argument that Horwich will win, I think that on paper Miller should still be the favorite by at least a bit, but I like Team Quest and I'm a Portlander which makes Horwich my home town guy. I also think his religious zealotry and crazy look is pretty amusing.