Victor Osimhen would benefit more from moving to Manchester United than to Chelsea, in my opinion. Chelsea's team has many good players who they acquired for a high price but still don't perform well, so if Osimhen joined the team, his career would likely come to an end as they would keep him down or refuse to start a game with him because they have so many players. Manchester United does have players, though, unlike Chelsea, who have players and still can't perform well this season.
It will be very exciting to see Victor Osimhen play for English clubs since it is ideal for players to play in many teams to demonstrate their strength and ability rather than remaining in one as a top player.
I actually think both teams are not suitable for Victor Osimhen if he is going to leave the Napoli club. Chelsea already has a lot of players like you said, but it is certain that they will be released and also some players who are on loan at other clubs will return to Chelsea, so their choice of players is quite a lot, but Osimhen will still be able to start there because considering what he has given it to the old club, it's just that it will be a question of whether he can appear directly together with the other squad, that will be a doubt. Man United looks like a good option but maybe he won't get the title in his first season here.
Manchester City will be the best choice but City already has Haaland but if the two of them can appear together maybe it will make City the most productive team by scoring lots of goals.
But it would be better for him to stay at his current club, because he is already compact with the existing squad and if he continues to impress the price will be even higher next season and playing for a bigger club will be easier for him to achieve.