You dont need to play as a striker to be clinical. You need to learn to finish if you are playing as an attacking player. Martial plays on the wing too but he is much more clinical than Rashford.
But martial is a better winger than Rashford is. Rashford from my knowledge was a average level winger in the reserves before LVG took a punt on him as a striker. His performances correlates with his career so far, his frustration/attitude & his progression.
LVG tended to want a particular type of forward - Rashford in terms of play style was what jesus is to City; a CF that can mediate around the wings & find the pockets of spaces whilst attacks happen through the wings and behind him. If jesus was played on the left wing or right wing - he would look decent but not productive enough & this is what I am seeing with Rashford currently.
Martial has the natural ability to be on the left wing but :
A) he isn't a winger so doesn't benefit the striker much
B) his style of play - his ability to hold on to the ball & his very close to feet dribbling style is suited to the centre
C) martial tends to cut in the same position as a LW with the intention to shoot or pass literally in the same way & can struggle to beat his man or be productive too
D) centrally he can hold on to the ball, play short passes or lay offs, dribble with the ball close to his feet directly face on with the goal as well as the defenders meaning his is able to better take on the defenders on both side since he has options to score or pass on equally on both sides.
Rashford has similar capabilities but nowhere near capable as martial on the wing due to the problems both of them have at being a traditional winger as well as Rashford'dribbling style & his pace is used to usually get past one defender centrally or two maximum before he ends through on goal - ideally his pace is meant to play on the defensive line to make runs in behind almost like Owen used to. Even scrapping that - the goals he scored in his debut season was centrally, predatory instincts being in the right place at the right time with a mixture of pace & precision.
Rashford is being held back by the formation, the key players in Jose's tactics such as lukaku, a lack of confidence in his own ability possibly as a winger and more probably now as a CF.
All in all, it's nothing to worry about - he is a young striker playing for a manager who's type of striker is fundamentally the types of ibrahimpvic & lukaku - he is more likely to utilise players like martial & Rashford as wingers because they are capable to do so & nothing more.
Rashford will be here longer than Jose & the way football is these days if Lukaku doesn't kick on under Jose - i struggle to see how he will be more effective than Rashford under most managers in the world.