I disagree. Who contributes more and can they do it against tougher opposition in the PL are the key questions.
Some stats on the two, and Sanchez for a measure of a top PL player:
To start, total contributions (assist or goal):
Mbappe - 29
Griezmann - 35
Sanchez - 41
How many contributions have they made where the team has won by two goals or more, vs. closer/tougher games, i.e are they just going to town on poor opposition and disappearing in difficult games:
Mbappe - 21 v 8
Griezmann - 21 v 14
Sanchez - 30 v 11
Another question would be maybe those big winning margins are all down to them and it's not fair to say they can only do it against weak opposition. When they score, is it the team doing the work or is it because they're just so good individually? So, how many of the games they contribute in are they responsible for 50% or more of the team's goals?
Mbappe - 11
Griezmann - 17
Sanchez - 22
What all that says to me is both put up good numbers but Mbappe struggles to score against tougher opposition, and when Atletico win Griezmann is often the reason why. Mbappe needs the team to dominate before he starts performing. He won't be the one to drag you over the line.
It also shows how screwed Arsenal will be if Sanchez leaves.