Saul or Neves, who would be best for us.
If you want a Fred with better heading skills and a few more goals in general, Saúl, and perhaps he could grow into a strong holding midfielder, but based on his play at Atleti, I'm not sure if he has the type of personality to be reliable there.
I don't know Neves as well, but if Man Utd really wants a holding midfielder for a 4-3-3, he seems to be the better type to get.
When using 4-2-3-1, I'd reckon you're better off with Fred - Neves than Saúl - McTominay, too. I'm not sure if a Saúl - Pogba could be better than Fred - Pogba or Pogba - Neves, either.
Maybe Saúl will benefit from the change of tactics and surroundings, but if he stays at Atleti I'd prefer him as a goal scoring offensive midfielder, left center back in a 3cb formation, or perhaps as a wingback cover. as CM/DM I'm not sure if I'd trust him enough, and I think that this is what United needs more than the things I just named.
I don't know if Neves fits well, either, but I do know that he'd complement Pogba and Bruno better in PES
(although Saúl would be the better option on the bench with his ability to play everywhere, to some extent)