Well of course, he's a superior player to Pedro. I just believe that in Pedro, we are acquiring someone who will suit van Gaal's 'philosophy' better, as he doesn't turn over possession as much, isn't as spontaneous than ADM and is more measured, calculated and disciplined in his approach. He really is perfect for the right-sided forward role, where Mata's lack of pace can hinder the teams.
So while di Maria is a tier above, I would rather have a team that are cohesive and perform better as a collective. Ultimately, better players win titles. I'm not escaping from that fact, but still, if the player fails to adapt or his attitude isn't good enough, he has to leave regardless of his ability. You can't really gauge how many points a player will earn for his team, as that transcends just goals and assists, but it wouldn't surprise me if Pedro contributes more to the team than ADM would have this season.
Speaking as if we've already signed Pedro here!