my vote would be mid march - spring breakers start coming a little later in march and the crowds can be spread out since different school systems have different break dates ....in addition, the prices are still not too high at the resorts if you can go earlier in the month and the flower/garden show is lovely...mid october can be packed around columbus day (october 10) and while the food and wine is fun, it really jams up epcot in the world showcase..that said, the halloween/fall decorations throughout the MK and hotels are so well done!
june would be my last choice - heat, crowds, cost of resorts-
you will have a great time no matter when you go ....let us know what you decide!