I think the most likely outcome of the fight is a defense oriented fight with both fighters relatively reluctant to commit. If everything goes right for Pacquaio, Marquez "training system" will get to his head and make him believe he can trade with Pacquaio. When that happens, it's game over.
Of course....he's an 8 division champion. He has seen every style and every trick in the book. Certain things give Manny problems, but nothing he hasn't been able to overcome.