Messi in very end of his career does not want to gamble with a strange coach whom he has not yet worked with before. So from this point of view, Pep is the best coach for Messi. Pep won't leave Manchester City so Messi will come to join the party with Pep.
I doubt that Messi can really help Manchester City. He is 33 and English football is different from the one he used to know in Spain. The game system would have to be adapted to him, which means that ManCity would have to change its current system. That would be bad for a well-rehearsed team. Besides, it would take at least a season to make it work. By then Messi will be 34, almost 35, and none of this will work.
I don't want Messi leaving Barcelona on a personal note/ground but I don't agree that Messi won't adapt to Manchester city. He had played under pep the in Barcelona and the football is not far off from Manchester city style. Ronaldo went to juventus and still playing good football -not at the peak as RONALDO- age but his game is good. Messi would still play good football even if he is to play on the moon, -he has nothing to prove -