I was there last May and also last Sept. Crowds weren't bad in May, but were even better in Sept. The parks were empty in Sept. (but the restaraunts were packed, due to the
free dining). There are trade offs for both.
May is prettier with the flower festival. I had no rain, and temps weren't bad.
Sept you get fewer crowds, free dining, rain every afternoon, hotter temps, and the parks are not as pretty. Sept also has a chance of hurricanes.
I enjoyed both however and this year am going May 21-28th and Sept 22nd-30th.