Wow, this is tough.
5, 10, 12, 18 months ago this wouldn't have been a fair fight. But, based on what we've seen from Zibanejad, this is a legit question ponder.
Zibanejad has impressed me tremendously this season especially, and I absolutely love his game. He's big, thick, strong, skilled and can score as well as he can pass the puck. He's also full of life and personality--which goes a long way with me--as I'm a big "intangibles/team" guy in general. I've turned into a big fan of Z.
As for Pettersson--damn, where to start? The kid has blown me away from day one. He's one of the players in the league--Makar, Dahlin and Q.Hughes being the others--that we don't know how great he can be. The term: "the sky is the limit" gets thrown around a lot but I think it's legit with Pettersson and those kids. I honestly think Pettersson has the talent to win an Art Ross in the NHL--he's that gifted. While Zibanejad is full of life and a happy-go-lucky dude, Pettersson is more reserved and quiet. That's not a bad thing, as Swedes are generally some of the best teammates to have, because their personalities allow them to fit into any locker room dynamic.
I honestly don't think you could go wrong with either of these guys. But Zibanejad is 26 and Pettersson is 21, so in this case I am taking the player who can offer me 5 more years of production, assuming both have long, successful careers.