I rate Rashford. I think he will be a good player and have a long career at United. Whether he ever becomes world class or not is hard to predict. He's a bit of a moments player right now. Flashes of quality with a lot of average stuff mixed in. Time will tell whether he can clean up the rough edges in his game and because a truly brilliant player. Either way if he never gets any better than now he'll still be a really good player for us.
Greenwood is a bit more interesting for me because he doesn't really have any rough edges in his game. Most young players have lots of good moments and bad moments, misplaced passes, poor decision making, loose touches etc. Mason doesn't really have this. Everything is just very clean. His touch is superb, his passing is crisp, accurate and well weighted with either foot, he makes the right decisions with the ball. The only times he ever seems to lose the ball is when he's trying to dribble past someone, which is expected.
That's a really good base to start from. On top of that you add pace, strength, extreme two footedness, excellent dribbling skills, powerful and accurate shooting with either foot and confidence and you have something potentially quite special. The ability to shoot equally powerfully off either side is huge for an attacker because it makes you almost unreadable for a defender.
I think he's someone who will walk into the team pretty soon and stay there because he does absolutely everything well. He doesn't seem to have off days with his general play. I guess the only factor I'm not certain of is how his pace will factor in to PL football. He seems very very quick and it's been claimed recently that he was potentially a very high level sprinter as a kid but it's difficult to really assess it from youth football and he's at an age where his real powerful running is probably still developing. If his pace turns out to be comparable with someone like Rashford I would say his ceiling is basically whatever he wants it to be.
Here's a video from a recent U18 match v Liverpool.