Rashford is about three times stronger mentally, has a passing ability that is way less erratic than Martial's, Rashford has displayed wider variety of skills like backheels, rabonas, flick overs than Martial who seem mostly rely on his direct running and pace to get past defenders. Rashford link up play is better as he tends to release quicker passes. It was there on display in the game against Chelsea when he was linking up with Lingard. Even last season he was linking up with likes of Fellaini only for the big guy to hit the cross bar with his shot. Rashford sends in much better crosses, Martial rarely crosses and when he does it's a random kick into the penalty area. Rashford has a good long range shot on him, has hit a good free kick very recently whilst the only time I saw Martial take a long shot was once this season when he hit the post. Rashford has more agility, better balance, more determination, better understanding of what's around him. Rashford closes down opponents better, chases long balls down and has as a result scored about three goals by simply closing down the goalkeepers. That also means he is a hard worker.
I don't understand how people can say stuff like Mourinho likes to give a youngster like Rashford a chance while doesn't like Martial for no apparent reasons. The difference in quality is very clear this season. It has more to do with Rashford's stronger mentality and self belief. Martial is more talented than Rashford if pace, dribbling skills and flair are the only thing you take into account. Rashford edges Martial if the season's output is taken account. But if we look at overall set of skill both players posses, it looks like Martial has the potential to be master of one, a beast at beating players and then cutting into the penalty area while Rashford could be jack of all trades. As of now, Rashford surely looks more talented than Martial. The fact that it may or may not change in the future shouldn't be a part of a discussion that talks about their current abilities.