Young has looked good so far, relative to his past play. He is in better position in help defense and has showed some potential in transition. I still think he has a long way to go with the mental aspect of the game.
Young can shoot the ball but isn't good at moving to open spaces to get quality looks, that is why his 3 point percentage is low despite having a good looking stroke.
RJ is already better at moving to an open space to get himself open, he understands where to be in help defense already.
The only argument that makes sense for Young playing over RJ is that Young has higher upside because he is younger, bigger, more athletic and has a higher release on his jumper.