Rashford and Martial both have similar ability and peak, and are both 20 goals a season forwards, at their best. But they are both wildly inconsistent. One week they are really electric, next week they feck everything up. While Martial goes missing in games, Rashford is extremely wasteful. Neither is significantly better or worse, and stats back that up, and anyone saying otherwise is just biased(which is normal, we all have favourites). Right now Rashford has more goals, so people are on Martial's back, but at the end of last season, it was the opposite.
They are both at their best when playing against a high line, and neither have that goal scorer's hunger, they are forwards, not CFs. Cavani is a pure CF, like Ibra or RvP.