PKing is vastly overrated, anyone can learn how to kill penalties. Defensive talent is a lot easier to teach than offensive talent. Ice time is completely the decision of coaches, so that doesn't prove anything either. Marchand has outscored Kessel at ES in the last 2 years, but he got smoked before then. If Marchand outproduces Kessel at ES next season, that will be completely valid, but I'd want to see Kessel when he's not playing for the Leafs or a Johnston coached Penguins. If Marchand outscores Kessel this year when Kessel plays a full season under Sullivan, I'd say that Marchand is better than Kessel. I'm not going to say that Marchand is better until that happens though.
The thing is that Kessel isn't limited defensively. He doesn't make any glaringly bad mistakes defensively just as he doesn't do anything amazing defensively. He's a net neutral defensively, which is what you want out of most wingers.