I think the thing about Markstrom is that he's split the season between being pretty good but underrated and actually elite. Pettersson has been elite all year.
It's real tight though, really Pettersson/Markstrom/Horvat/Edler have been carrying the team this year. Pettersson for our offense, Horvat for eating up tough minutes while putting up points/solid defensive play while playing with bottom 6/AHL players, Markstrom for being a wall and Edler for quite clearly being our #1D. And this team is still bad.
Lose Markstrom and we're in bad shape. Lose Pettersson and our offensive becomes Horvat/Boeser, lose Horvat and we'll have some real bad players getting real tough minutes, lose Edler and we've seen what happens.
I think if Markstrom played this way for the whole year he would be #1, but with Pettersson playing like an elite player since game 1 I think you have to give him the edge.