The Champions League is a separate event and the mood of the teams can change dramatically on the eve and during these matches, so I would not immediately give preference to Manchester City. Firstly, the game will be held in Paris, which may give some advantage to the Paris club. Secondly, willy-nilly, Paris Saint-Germain is slowly rolling into the season and the club's game should also gradually improve. Just from the second point of the story, the third point arises that Messi should slowly join the team and bring more benefits to the club's game, which can have an impact already in this match. By the way, City's game at this stage is also not perfect, so the prerequisites for fighting in a single match seem to be present.
Clash in early of Champions League and in group stage is not much exciting. Purposes of teams are getting tickets to the next round and it is best if they can end in first position of their groups. However, it is not a mandatory mission and sometimes ending a group stage in second position can be better.
Messi need more time and more matches to really become a part of PSG team. He does not have good connections with other players in PSG and time will help him to build it up gradually.
This match will be interesting because Messi will meet Pep and fight with Manchester City, the club that wanted to have him very much. Finally, Messi decided to join PSG and rejected chances to reunite with Pep in Manchester City.