My view as an outsider is that Martial is the more talented player but Rashford is more likely to be a club legend for United.
He loves the club, has more heart and a burning desire to do what it takes to improve his game and iron out any flaws.
I don't see Martial at OT beyond about 3 more years no matter what but I cannot imagine Rashford anywhere else.
I've seen this sort of theory around, and probably believed it myself for a bit, but it does beg the question of 'why'?
Like, personally, I don't think it's much beyond style of play. Rashford is 'busier' in the Vardy mode, maybe it seems he cares more. Or perhaps the fact that he's had cameras and media in his face doing loads of interviews in English talking about his 'story' and determination.
He may we'll be more driven than Martial, but I can't see it being more than a guess. I think if their roles were reversed and Rashford initially moved away from home at 16 to play for, I dunno, West Ham, and then went to France or Germany at 19 to play for a big club, I imagine that would be seen similarly impressive and dedicated. It's just the norm here for others coming in - but when things have gotten tough for Martial, there has been no suggestion he has run and hidden or agitated for a move. He has quickly quashed every rumour about him wanting out, even as a sub. I suspect that stereotypes would always see him as the lesser dedicated. He doesn't run all over the pitch, and doesn't speak to the press much. If he did, he could be constantly telling his story of a tough upbringing in France and how 'scary' it was leaving at 19 and people would think differently I suspect.