I've always thought that Pavlik's offense was brilliant, but his defence was shit. That's essentially the worst thing you can be against McClellan, aside from having a shit offense and shit defence. McClellan and early is my pick. I think it should be said, that Pavlik's offense is likely the second best of any middleweight this century, obviously after GGG. Granted, there hasn't been too much competition, but it's really fucking good.
I think it all comes down to how much damage the frontrunning McClellan can do in the early rounds - he may stop Pavlik early or at least do so much damage to him that he can't recover. If not, however, I think Pavlik can potentially wear the G-Man down and stop him late.