I'm also of the opinion Martial is the better player. I am similarly of the opinion that Rashford is a fantastic talent too.
The impression I get though, is that Martial doesn't really have a real chance of progression into being an automatic first XI player, whereas Rashford does. They will continue to be 'rotated' for now (i.e - Rashford starting majority of games and Martial being a sub) but the fear and expectation from me is that, at some point, Martial will stop doing what he's been doing with his 15 minute cameos and Rashford will pull away, with perceived justification due to Tony's recent contributions. Basically, Rashford will have to play a lot worse for a lot longer than Martial for the latter to be an established first XI player, which is unlikely, as Rashford himself is also a top player.
I don't think this is the right order of things personally. I've never seen a caf go on about 'movement' and a primary objective of 'tiring defenders out' between two players so much as this season when the great 'Rashford va Martial' debate has developed. For me, Rashford is by far the busier player of the two. That is likely preferred by Mourinho, and also by English football observers in general who hold 'work rate' very high. Rashford is perpetual motion, which almost looks 'dangerous' in itself when you do it as fast as he can. He can get people off their feet just by running in behind without the ball. Martial is more continental in his style, and more ball oriented. This leads to praise with the ball, and criticism without it. Rashford has 'better' (or simply, 'more', in my opinion) movement than Martial. I do think Martial has very underrated movement though. He's, again, perhaps more continental with it. Rooney ran everywhere, left back, right wing etc. Not necessarily 'great movement', and as a forward player, his movement was not necessarily 'better' than RVN or Hernandez, despite moving 'more'. For me, Martial has more of this kind of movement. That of a striker, perhaps, and not a winger. He's not static in and around the box at all. All of his goals this season have shown good movement to make himself available for the right pass. The last two goals have been from give and go's with Lingard and Rashford respectively - but he didn't lay the ball off and stand still. He was on his bike and got it back through on goal. Another example was his equaliser against Boro last season, running off Zlatan into space to receive the knockdown. What he doesn't do is position himself to receive 30-60 yard passes in the way Rashford does, which means he doesn't possess the same threat in behind on quick transitions. Within a shorter space though, I think his movement is fine.
Said before, but I can see Rashford developing like Bale, perhaps even slightly more rounded. I think he'll have a similar physique to Welbeck, and if by that time, his game is put together, he will be a force of nature. Martial, while possessing physical gifts, will never rely on them in the way Rashford and Lukaku do I don't think. He needs the ball more to his feet.