Both Manchester United and City need wingers, and Mitoma will be fit in both clubs, just that the player will be the one to decide which club he will prefer to join in the January transfer window, that is, if Brighton will be willing to let him leave in the transfer window.
Rather than looking for a new winger, it's better for Manchester United to sign a new goalkeeper and few defenders. Their midfielder and striker already strong enough, so they just wasting their money to sign a new winger.
A player will join a big and tough team, Manchester United only a big team, so Mitoma should be in Manchester City.