To be fair while he's perhaps not better than Hernandez overall, he's certainly better in this Utd team. Hernandez in my opinion was an incredibly infuriating player because he was simply brilliant in and around the box but the rest of his game left a lot to be desired. I think Rashfords hold up play is already better or at least on par, although I do still see this as an area he needs to improve upon but that should come as he physically develops. Rashford also seems to me to have more blistering pace, al be it that Hernandez was no slouch but he very rarely committed players through pace and dribbling in the way Rashford has done. In the box Rashford has a way to go to catch Hernandez, he's shown glimpses of great movement I.e his villa goal but Hernandez has consistently been one of the best for in the box strikers for awhile now.
I certainly believe that Rashford has more tools in his pocket to become a better player than Hernandez who despite being a great goalscorer is an otherwise very limited player. But of course it remains to be seen how well Rashford physically matures and whether he can maintain and build on a decent technical game.