Well. I don't say that there is a big gab between them when they are both playing their very best hockey, at least not right now, but to say that Marner somehow has a higher level than Pettersson in general is a bit of a stretch if you ask me. What's so special about Pettersson is that he excels at every aspect of the game. Shot, playmaking, speed, hands, defence, iq, mental game. He pretty much has no weakness except for his size at the moment. And since he is 100% of the time a top focus of the opposing team he needs to fill out a bit more and find a way to overcome constant pressure and bully from shutdown players. He cannot be completely neutralised since he draws so many penelties and is such a great force on the PP but still.
He is on his way to overcome that already. This playoff is proof of that to some degree, even though he has been "stopped" a few games here and there.
If he can overcome this "only" weakness (there has to be more im sure), he will pretty much be the most complete player since when exactly? I donno. Absurd potential.
As for how he compares to Matthews. I think it is too early to tell. Matthews is special in his own right. Pure goalscoring talent. One of the most talented goalscorers of all time no doubt. I could see myself picking anyone of the two, which in all honestly speaks in Matthews favour since I'm Swedish and a bit biased i guess.