At United and at City, Mangala can command upwards of £100,000 a week from the get go.
He's guaranteed a starting birth, as both teams need a monster of a CB. At City the pressure will be on right from game 1 of the season, he'd be expect to win leagues and perform to a certain level consistently, otherwise he'll be kicked out asap. At united, they're in transition, which naturally brings pressure, but not as much. Finishing 5th will be an improvement on last season, so the expectations won't be as high as City's.
I mean, he might go to United because he's a fan, or he's got a hard-on for some of their players: Rooney, Van P.... or he could be mentally weak, and would prefer a gravy train of a job, knowing full well the media will analyse the CM area than his own.... rather than instant spot light that City brings.