To replace Pogba, I would prefer a goalscoring midfielder. Someone like Havertz (17 goals) or Fernandes (20 goals).
We can buy one of them and another midfielder like N'Domebele.
Maddison would be a good signing, but he wouldn't be my first choice.
My priority signing would be a RW. We definitely need someone like Sancho, Chiesa, Pepe to improve our attack.
Pogba and Maddison both scored 6 non-penalty goals last season. You wouldn't be losing out on much there.
Pogba got marginally more assists (9 to Maddison's 7) but actually made fewer key passes. It's arguable which is the more creative. Again, there's hardly anything between them.
That being said, Pogba had a couple of better seasons in Italy. But that's Italy... I'd take that period with a pinch of salt when judging his suitability to United.
I think the two of them are very similar offensively. The main difference between them is how they defend. Pogba plays deeper so gets a little more involved physically (though he's not very good at it), whereas Maddison works harder in a high press system.
More importantly for the current dressing room, Maddison is a United fan who would run through walls for the club, whereas Pogba can often be smiling and laughing in defeat. The latter doesn't have the never-say-die of a Fergie disciple.