A lot of punches would be thrown, both have a lot of pride. But I think Spence wins this because he has a better ring IQ. And he's not as sloppy with his punches as Williams is. Spence wins a commanding UD.
That last sentence was uncalled for. But I agree Spence would have beaten Tall Paul. Too accurate, and despite being shorter, physically stronger than Williams. I think Williams goes the distance, but he'd be taking a whuppin late.
If Margarito can break Williams down enough to lose a close decision I'd expect Spence to win clearly or by late stoppage.