It would be great to watch Mbappe at Manchester City. I'm sure every Manchester City fan would want that. But we know that Mbappe had some problems before when he was in the same team with Neymar and Messi. He might have similar problems playing with Haaland. There are younger players at Real Madrid, so he can gain respect from them. And we can see him as a dominant character at Real Madrid. But when he plays with Haaland he might not be the dominant character. I think he will be more successful at Real Madrid.
Man City fans must expect this but I think it is unlikely to happen. If Mbappe joins Man City, he won't play in the striker position because Haaland is the main striker of Man City. Mbappe must be ready to play as a winger, he must serve Haaland. I don't think Mbappe wants do this, he must expect to be the main striker in his next team. He also must dream to be the main goal machine of the team. So, there is no way to play together with Haaland. It is still better to move to Real Madrid because he will play as the main striker there.
Sure, if he is in the same team with Haaland, there must be a competition to be most shining player there. I assume Mbappe and Haaland can be difficult to cooperate together because they want to score as many as possible. It will be bad for the team if the player become egoistic and play individually. So, it won't be a good idea to join Man City.