This is the first season that Martial has gone through a barren spell, something that all strikers go through so I don't agree with him being a very streaky player as you mentioned. He's also heavily involved in our buildup whether he's scoring or not, something that tends to get ignored. Also I don't think he's that similar to Rashford other than in their ability to produce a moment of magic out of no where. Rashford is suited to the wing, where he can use his explosiveness and dribbling in less traffic while running into space.
Martial, on top of his technique is now built like a proper center forward, and is more suited to playing in the middle than ever. He's fine with receiving the ball in traffic and 8 times out of 10 will retain the ball under pressure. He's a willing passer and rarely selfish when presented with an option to advance the ball, and is a better finisher. It's also not true that all he does is come short for ball to feet either, he constantly varies his runs to get in behind, and is adept at timing them to stay onside. Mason is tidy on the ball and a very good finisher as we've all seen, however the rest of his game is still raw and I believe a stint on the wing will be good for his development like how it helped RVP improve his all round game. They're all at different times of their development anyway and I don't see the need to constantly lump them together.