Marquez was juiced up.
Horn actually lost that fight. Go back and watch it again. He got a hometown gift decision.
Spence is a counterpuncher, which gives him an advantage, since counterpunchers have the best chance against Pac. However, Spence is not able to adjust during a fight like Floyd could (that's why I refer to him as robotic). Mikey Garcia exposed that when he lost to Spence. Pac will use his superior legs to control distance on Spence. He won't be strong enough to really hurt Spence, but it won't be necessary as long as he keeps scoring. It will go 12 and it will be close but I got Pac by close decision.
If were a betting man (which I am not), then I would probably also hedge that bet with Spence by KO, just in case Father Time does indeed kick in on Pac.