Hi Kimberly,
Thanks for visiting the Disney Parks Moms Panel with your question.
You are choosing between two great times of year. The weather during both September and May in Orlando is warm, but not as hot as it can be in the middle of summer. Since many children are in school during those times, the crowds will typically be a bit lower. I think that the crowds during the two date ranges that you mention will be very similar. The September week may be a little bit less crowded, but not so much that I would use that as a basis for the decision. When I am looking for predictions of crowds at Walt Disney World, I like to visit the unofficial site touringplans.com. For a fee, you can get access to their predictions for crowds at Walt Disney World for a year.
You might also consider the special events that may occur during each of your possible dates. In May, you would be visiting during the
Epcot International Flower and Garden Festival, which includes amazing landscaping and topiaries, delicious food booths, and the Flower Power Concert Series. Although official dates have not yet been announced, your September dates may include dates for
Mickey's Not-So-Scary Halloween Party, which is a really fun nighttime event at Magic Kingdom including trick-or-treating and special entertainment.
I hope that these thoughts are helpful for you, Kimberly. Whichever dates you choose, please do not hesitate to come back and ask any further planning questions that you may have. We on the Disney Parks Moms Panel love helping you to plan magical vacations!