Yes silva and aguero's legacy will not be the same as if they had achieved the same feats at a club like real madrid, barca, juventus, milan clubs or even liverpool, there is a different type of pressure and scrutiny that comes from having to perform for these type of clubs.
Man city are a small club and I'm not saying that as a dig towards them as united fan, it's just a reality and I wouldn't not being saying this if he was joining liverpool, in some ways it will be a sad sight to see man city's stadium as potentially messi's final stage to perform on whilst still being able to deliver world class performances, it should be something far more grand or historic stage for his last remaining years at the top.