It comes to the point, for quite some time too, its fair to say City have noticeably better pull on players than United.
We winning things, fan base power is jumping, best facilities, reputation for best coach and staff and so on. United pays huge amounts on Eredivisie players (Antony, Malacia), few very good players but undoubtedly post their prime (Varane, Casemiro, Eriksen, Ronaldo), few that looked even quite desperate (Evans, Weghorst, Amrabat, Mount), loosing competition for best players quite often.
Currently nobody believe United can attract best of generation level players like Haaland, Bellingham or best in PL like Grealish, Rice or best world class on their position prospects like Gvardiol, McAllister, Nunes, Enzo.
Meanwhile City looks to be set in a bracket with few best and most famous teams behind maybe only Real. Barca has its strong, historical pull (with important factor of place of living), but it feels its declining a bit due to economical reasons (players would have second thoughts, asks for guarantees etc).