I guess it will come down to Messi's personal desire to how far he wants to enhance his personal legacy and have different life experience. I'm not quite sure how earning abit more money for 2 years changes his life.As a legitimate business project. On a personal level, for Messi himself and his agent, City or PSG can give him more money under the table than he will ever be able to count.
I'm sure he understands the difference between PSG/City and Manchester United in terms of historical and cultural equity and the potential reach each club provides him. Eg: if he joins PSG, most of the news reporting of his PSG career will be in French language, which dramatically limits its accessibility to global audience. Thats been a major issue for him, in terms of global exposure, whilst in Spain. Also if he is after a new life experience, why join up with his ex colleagues at City. Seems like a very boring and overly safe option.
If he wanted to be remembered at the best player that ever lived, Manchester United provides him that platform better than any other club available, given the status, scrutiny and exposure that he would enjoy.