To compare Rashford and Abraham position-wise is futile, as lots of you mentioned they are different players. BUT, a comparison is warranted in terms of attacking efficiency, or threat, as they both play in attack. The idea that the striker outscores the inside forward is outdated, as in modern football both positions share the same amount of responsibility for goals and assists.
In PL last season Mane and Salah scored 22 goals and Aubameyang and Aguero 22 and 21 respectively. In La Liga it was always Messi and Ronaldo the top scorers each season. In France Mbappe and Pepe were top scorers and maybe even Neymar would be top 3 if he was not injured for most of the season. In Bundesliga and Italy, the strikers generally have more goals, but the wingers are very close to them in terms of output.
And that is only goals, if you take assists into account you will see that in modern football wingers have even more responsibility for the attacking potential of a team.
Now Abraham showed what he can do when in form. Is he world-class now? No! Can he be considered world-class in the future if he produces only half of his current goal/minute output? Absolutely yes! 7 goals in 5 games are too much for Aguero as well. Only time will tell what is Abraham's ceiling.
Rashford, on the other hand, is not breaking through now, he is an established player at a big team with 3.5 full seasons under his belt, playing in PL, CL and in world cups. Is he world-class? No! Has he the potential to become one? Maybe, but after 3.5 full seasons, you can kind of see the limitations he has. He is a good player, but not the guy to take United to the world-class level where it once was.