Generally speaking, I feel Martial is the player we should be playing on the left. Martial is definitely in my eyes the player with the greater potential of the two. I still can't really understand why it is that people such as Ronaldo and Gerrard seem to over-estimate how good a player Rashford is. For example, Ronaldo put him in the same bracket as Mbappé and Asensio, which I thought was somewhat wrong. Gerrard also said that he loved him as an English player and feared him as a United player which again speaks to the fact that young English players tend to get over-rated.
Anyway, Martial is definitely the better dribbler of the two. His dribbling was sub-par last season but seems to be back on the right track now, although he still has a way to go to recapture his first-season dribbling form. Mind you, his dribbling seems to be a function of his confidence, so it seems logical that if he continues to put in improving performances, his dribbling form will take off. Rashford lacks Martial's power as a dribbler and moreover, you never really get the sense that he can turn the opposition RB inside out.
Secondly, Martial seems to be the better finisher of the two. You always get the sense that when Martial finds himself one-on-one with the GK inside the box, he is going to calmly side-foot the ball into the net with the inside of his right foot. Rashford on the other hand seems to lack the same composure in front of goal, like when he missed as good chance in front of goal away to Swansea for example. Rashford also seems to have less clarity of though when shooting from out on the left. With Martial, you know that he is going to cut inside onto his right foot. Rashford however seems to suffer from having too many choices in his head. I know Rashford has three goals in his last three games, but only one of those goals was a properly composed finish (Vs. Leicester).
The only real facet of Rashford's attacking game that is noticeably better than Martial's is in terms of making runs in behind the defence. However, this kinds of leads you to think that Rashford should be playing as a ST and not as a LF. I feel that he is rewarded more for his intelligent movement when he is a ST as opposed to when he is a LF. Martial seems averse to running in behind the defence, which is disappointing as it's not really that difficult to master from LF compared to how difficult it is to master from ST. To be fair, this seems more like a lack of application on Martial's part more so than a lack of ability.
Ideally, I'd prefer if Martial started 2/3 of the games at LF, with Rashford starting the other 1/3. Also, Rashford could play 1/3 of games as a ST, with Lukaku starting the other 2/3. I think this would be a good rotation system, with the exception of injuries, which would enable us best to challenge for the PL and to go relatively deep in Europe.