For Messi's legacy you best hope he doesn't move to City, can you imagine a 34 year old Messi moving to City and being outshone by KDB for example? Especially when 36 year old Ronaldo is still doing the business and the main guy at Juventus.
There's always been a big question mark about Messi and his ability to play in another league, but it's no good deciding to do that now, should have done it 5 years ago, so if i was him in terms of legacy I'd stay well away from the mighty PL, it has potential to chew him up, spit him out Di Maria/Thiago Alcantara style, if that happens, well you all know me I'm a fair guy, i'll give him time (say 2 months) to settle into his new club, if KDB is outshining him or any other of City's titans after 2 months I'm going to be like "get that flop out of here".
Stay where you are Messi in the Cage Rage league looking good against plumbers and doormen, come to the UFC (PL) at this stage of your career, it's not going to end well for you.