I think that Messi should think of Manchester City as a second option after being free agent in the summer. He would surely prioritize returning to home, Barcelona. However there can still be some hindrances for Messi to go back there. FFP rules in the La Liga can block the way for Barcelona to make this transfer happen. Otherwise Barcelona also wants to sign him again and Xavi also agrees with the board.
If the deal between these two sides doesn't happen then I think Messi should think of Manchester City before PSG - I guess that he wouldn't like to leave Europe soon. Maybe he will need to adapt to a new team again but there is a big difference at this point. The difference is that he can adapt much faster to Manchester City's game plan compared to his first times at PSG.
Because he is already very familiar with Guardiola's system and he can feel like he is playing for Barcelona nearly.
City will not get him, Messi is great and all but City already has a great team and they will probably not go with Messi because it would take time to adjust and he is getting on the older side as we can see. PSG is a good enough team to stay if you ask me, and a lot of people are making a noise about this like he will 100% leave but I remember that everyone talked about that for him leaving Barcelona for 10 years as well and he didn't up until Barcelona couldn't afford him.
This is why I feel like there is not going to be an issue and there will be a lot of time for him to decide but he may stay with PSG and try to win UCL with them, they may have been unlucky so far but a few good midfield and a good manager could see them winning it without a doubt.