I agree that you will probably not notice much of a difference between the two weeks. We usually go in May and have been both of those weeks. Last year, we went the last week of April/first week of May (Easter was earlier), and some days we walked on rides and others the park was packed. As long as you have a good plan and utilize fast pass, you will be fine. We have always gone in May, but due to commitments this year, we are trying September for the first time.