I went in both May and October last year and enjoyed them both. October was the definite winner for me, though. These are the pros and cons of each, as I see them.
May Pros:
*Star Wars Weekends at MGM (with special limited-time-only pins you can buy and Star Wars characters roaming around Star Tours doing 'impromptu' scenes, not to mention the block party)
*Flower and Garden Festival at Epcot (with daily concerts of 60's musicians and the ladybug and butterfly releases)
*Warmer weather, if you like that sort of thing
May Cons:
*Hot weather a good bit of the time
*A little rainier than October
*A good bit more crowded, especially around Memorial Day and late May
October Pros:
*Mickey's Not-So-Scary Halloween Party! (Trick-or-treating all over the MK for both kids and adults, big Halloween parade with a long villains segment, Halloween Diamond Horseshoe show, and more)
*Epcot International Food and Wine Festival (begins in late October, booths set up all over world showcase offering snack-sized tastes of all different kinds of food and alcohol from all over the world for $1 - $4 each, and beer and wine tastings)
*Smaller crowds, shorter lines
*Less rain than May
*Cooler weather
*(Halloween Horror Nights at Universal Studios, if you're willing to go)
October Cons:
*Can get a bit nippy at night
*A little more likely to have rehabs (though this wasn't a problem for us at all--check the rehab schedule)
I'm sure I'm forgetting something, but this is how I remember my trips. Halloween is my favorite holiday, so I lean even more heavily toward October. Hope this doesn't confuse you even more.
*Ethyl*