Alistair Overeem is a strong, technical striker who has the ability to bully lesser fighters around and abuse range with his kicks and staggering striking. He packs a brutal punch and has the ability to finish most. He definitely could beat Cain, utilizing the clinch, knees and range.
However, Cain is the quintessential intelligent mauler, relentless, enduring. He'll keep the pressure on Alistair, staying in the pocket, not allowing Alistair to dictate range, hitting him with the kitchen sink early, then taking him down and pounding on him perpetually.
People who claim that Alistair will use his knees to great effect don't realize that throwing said knee will assist Cain in getting him to the ground. Once there, Alistair's inability to take punishment, endure an onslaught (which Cain will undoubtedly bring) will show itself and Overeem will wilt.
This is the same man I've seen tap to strikes from light heavyweights. Arona, Rua anyone? He's been frozen by straight jabs from Werdum. He's not fighting a one dimensional guy right here. This isn't Lesnar. These aren't underwhelming journeymen like Duffee, Rogers or a guy that is past his prime like Cro-Cop. This is probably the best fighter in that weight class in the world right here.
I fully expect Alistair to quit. He's a front runner. He has the tools to win, but he doesn't have the toughness, heart or cardio to outlast what Cain brings. He will give up. I've seen it before. Adding muscle doesn't change a fighter's mental make-up.