Maybe objectively you go Matthews. He's more proven, he's already filled out to a man's body, he's a ridiculous level goalscorer, who will probably win multiple Rockets, and he's a better all-around player than a lot of people give him credit for.
That said, as a Canucks fan though, I wouldn't trade Pettersson for him. Yea part of it is becoming attached to the player as he grows, but he's a legitimate choice here as well. The underlying metrics show his effect on GF% is extremely high, and watching him in these playoffs you really see how much of a gamer he is and how willing he is to push through physical play and take abuse to make a play. He needs to get stronger and there's no way to know how much he'll be able to get and whether that will affect other parts of his game, but I think he'll end up fine. As it stands, his vision, playmaking, shot and transition ability are all top notch. He backchecks like a madman, and looks like he has the potential to be an elite 2-way player. His play still screams Datsyuk whenever you watch him, and a lot of the underlying numbers like him in similar ways. He even has the potential to be a decently better goalscorer than Datsyuk.
Matthews is awesome and will probably go down as one of the best goalscorers of all time. He's very similar to a young Stamkos, except he's a better 5v5 goalscorer, and I think a better all-around player. But just as I would take Datsyuk over Stamkos, I would take Pettersson over Matthews here. Pettersson has more risk in reaching that level than Matthews has, but seeing his drive and competitiveness, I think he gets there.