Matthews' 4 goals in his first game isn't particularly relevant to him having an extra gear. That was a great first game, and it will be a moment that people bring up in hockey trivia for a while, but it isn't an example of anything except that he had a great first game.
Edit: Personally, I think MacKinnon will be the better player when it's all said and done. Development isn't a straight line. Players develop at different rates, and they play in different environments that can contribute to that development. Maybe MacKinnon doesn't repeat this season, but I don't think he has to if he's going to stay the better player.
I think MacKinnon's skillset is such that he could end up with multiple 90+ point seasons, and I'm only saying 90+ points because it's so difficult to have multiple 100 point seasons. It's a very short list of players who I think can accomplish that, and MacKinnon happens to be one of them.
Matthews, to me, is more of a 40-35/45-35 guy at his best. He's a great player, but he's a level below MacKinnon.