I have a feeling that Errol can get it done late. Problem is we haven't seen Spence go into the championship rounds before. But with Brook bulking up and then coming back down, I think he's going to feel weak after 9 rounds and if Spence can step on the gas in 10/11/12 the chance is there.
Early on though I think Brook just out boxes him. Better head movement, he creates better angles, etc. I expect to see Errols head popped back a few times. His defense so far has been the most questionable part of his game.