There is a big big difference, Mbappe has a much higher ceiling imo, and is already much better in every single aspect of his game, Rashford is a big talent in his own right but not comparable to a player who looks like he would have relatively failed if he doesn't at one point in time become one of the handful of great players in the world.
Its hard to say where their respective ceilings are after only a year or so worth of performances. A lot also has to do with work ethic, attitude, and coaching.