Both would likely get KO. I actually think Spence would put up a better fight than Thurman. Thurman may look good and present Canelo with some small problems with his speed and footwork, but he would imo succumb to Canelo's body attack and precise punching and timing. I see Spence landing harder leather than Thurman on Canelo, but lose the battle of body shots and eventually head shots. Canelo is a bit more complete than Spence. Canelo has great defense, very good speed in combination punching, Explosiveness, Size, Power. Canelo is a bit too much for Spence.
Why are we talking about WWs fighting a MW?? Canelo beats Thurman and Spence on the same night. Too big, too strong. That's why there are weight divisions
You got a point about the styles. Spence doesn't move laterally much. But also factor in Alvarez has displayed poor workrate against fighters who throw in high volume. GGG and Trout come to mind. Spence is another guy with a high workrate. It be worth seeing assuming Spence cleans out the ww division.