If we see the unmotivated, out of shape Rampage from the Bader fight, Shogun will win hands down.
If we see a Rampage that's in shape, I could see this going to a decision, though probably still for Shogun.
I honestly don't really see why people put Shogun in such high regard in UFC, he hasn't really beaten anyone of note other than Chuck and going 1 and 1 against Lyoto and Forrest. That being said, I'm rooting for him over Rampage.
I'm betting on Shogun by punches followed by a RnC in the third.