Mayswon a rookie of the year in ny, an mvp, a world series, plus a few other major awards. If a player can be honored in 2 cities, he's got a good case. Mays would not hold the career stats but for his nyc time. To me, its more the different franchise than his ny performance.
All of Mays accomplishments were made in the Giants franchise. He played two years at the end of his career in the Mets franchise. Even if you took his four great years for the New York Giants into account for the Mets they're not going to retire his number for that. Without all the incredible years he had in San Francisco they're not even thinking about retiring his number.