Good luck with that, Mbappe is obsessed with improving. The guy won the Euro and Ligue 1 and went straight home to rest so he could practice the next day while his teammates celebrated. Rashford is a hard worker too, but he won't outwork Mbappe. Only an injury or some major changes in his life would change his progression.
Also, people saying Mbappe is already the finished product and won't improve much are crazy; the kid is 18! He has very solid foundation, but there's a ton of things he can improve, such as range of passing, aggressiveness, strength, technique, set-pieces, and headers. That's not to say he's bad at any of those, but that's definitely aspects of his game that will improve over time. His finishing while great already will only get better over time as well.
I do believe Rashford has the most room to grow out of the two because he is fairly behind on certain key aspects such as finishing and decision-making. If he can sort those two things out (and it's not a given), then I believe Rashford will be a world-class player as well. Right now, I think it's an absolute joke to compare the two, and it's probably only in a united forum that you will find people willing to make that comparison. I strongly suggest anyone who disagrees with that to watch full games of Mbappe instead of just highlights, it's pretty glaringly obvious right now who the better overall player is.