Prime for prime I would say that Khan has a slim advantage as far as power goes. However, Brook has some as well and is willing to trade. But since this is a retirement fight for both, I don't see Khan power no more. He can get to Brook but Brook has more chin than him. It's the question on what will happen to Khan if Brook will hit him flush in the face, chances are, he might go down.
After that exposed Khan needs to work more with his good defense, Brook's will try to penetrate and knock him down by hitting his chin. It's a must find a way to avoid and to make sure to throw good counter to avoid that knock out punches to hit him. I would like to see aggressiveness from Brook, though. If he will start strong and try to corner Khan, still anyone's fight here we don't know if what would happen till we seen them both inside the ring..