I think that the first thing we should do is avoid constantly comparing the two and mentioning both their names in the same sentence. IMO, comparing them constantly gives off the impression that they are both of equal ability, which I think is not the case. For me, Martial has definitely the better technical skill-set, whereas Rashford's general technical ability tends to let him down quite regularly.
In terms of their respective ceilings, Martial's, I feel, is definitely higher. I wouldn't go so far as to say that he is a potential Ballon D'Or winner, but he has unquestionable top-level talent. At his hypothetical peak in the near future, I could see him breaking into the Top 10 of best players across the world, although not into Ballon D'Or territory. For this to happen though, I feel that he needs to play as the ST, which will be very problematic as long as Lukaku is at United. That's not to say Lukaku is a bad player; it's just that Martial has the higher potential out of the two as a striker due to his more refined technical skill-set. Also, playing Martial centrally would make him much less predictable in that he would have license to roam deep and wide, and to form over-loads in other areas of the pitch. His recent misses against Bournemouth and Leicester aside, his finishing tends to be highly clinical; playing him central would also likely lead to Martial getting more scoring chances than he does as a LF. In his first month at United, he showed himself to be very capable of receiving the ball to feet with his back to goal, which is obviously another important trait of a CF.
As for making the next step, Martial has the ability to do so. However, I feel there is more to him making the step up than just being played out of position (Although he's far from an egregious LF). His speed of play is definitely slower than it was in his first season at United; as for why, I feel it is a direct result of his lack of confidence. IIRC, I read somewhere on here that before the Leicester match, he had 6 goals on the season out of 11 shots. 11 shots!!! Taking only 11 shots on the season is shot-shy territory; the only reason why Martial should be shot-shy is if he is suffering from a lack of confidence. , My guess is that this lack of confidence is strongly correlated with José's presence as manager; therefore, I find it hard to envisage Martial making the step-up as long as José is manager (Irrespective of whether José is the right man or the wrong man for United in the long-term).
Rashford on the other hand does not have the ability to crack the Top 10 of best players in the world. For United to get the best out of Rashford, I feel that he has to be played as a striker in a counter-attacking set-up. Rashford's greatest strengths are his movement, pace, and runs in behind the defence; his biggest weaknesses are his lack of dribbling ability and lack of technical control on the ball. Asking Rashford to play as a LF is negating him simply because there is less scope in wide areas to make runs in behind and to cause problems for central defenders with his movement than there would be centrally. Furthermore, asking Rashford to play in a possession-oriented set-up is counter-productive, seeing as he lacks the technical refinement to be significantly involved in either the build-up or in intricate passing. As a ST in a counter-attacking set-up, Rashford would be at his most effective; his ability to find holes between the FBs and the CBs would severely hurt the opposition defence. Therefore, in terms of stepping up, we have to be cognisant of Rashford's strengths and weaknesses, rather than consistently forcing him to play out of position.
