According to the planning books I have (Rita Aero's and the Passporter) July 4th is as crowded as Easter and Christmas. Memorial Day is comparable to Thanksgiving and less crowded than the first option and it appears to be pretty comparable to "regular" summer crowds. July 4th would also be a little hotter. Remember too that usually when it is more crowded the parks are open later to compensate and usually more parades and shows scheduled (never a sure thing the way the hours have been lately but it's usually this way). Have fun!