If you're going to go in May, definitely choose the first half of the month. The second half gets very crowded with school groups I've found. The weather is beautiful in May (though it's starting to get hot then) and perfect for swimming and flower viewing at the Flower and Garden Festival. December is fun as well, though, with Mickey's Very Merry Christmas Party and all of the holiday events going on. If you're big swimmers and sun worshippers, I'd say go with May. If you spend most of your time in the parks, I'd say stick with early December as I find it to be much less crowded.