I have the feeling that none of them will become a world class striker / number 9. They will stay that way until the end of their careers. Different managers will just casually play them on the wings - left, right, some games as strikers, false 9s etc. There will be good games and bad games but they will be inconsistent as ever.
Dont know how to explain it but they both lack something - be it decision making, awarness, concentration, anticipation or football intelligence, its something which the best strikers have and Rashford and Martial dont. For example look at old videos of prime RVP, Falcao, Cavani, R9, Ruud, Etoo etc. and ask yourselves do you really see Rashford and Martial reaching such level playing as strikers / n9s ?