You have proposed my 2 favorite times to go, so from my perspective it's a win-win
. Having said that, here's my thoughts on the 2 times, in case any of it helps you:
(1) From a "mental" standpoint, I prefer May to September because after a long winter it's nice to look forward to Disney. September is just so far away from winter, and by the time you get back it's time for winter again and you don't have another trip booked yet because you just got back
(2) From a crowds perspective, the crowds are better in September than in May. We have always gone in early May, though, so the crowds aren't bad. But in September, the parks are dead if you get there at Rope Drop.
(3) As far as weather, I prefer the weather in May. May's weather is a little milder than September. September can get very, very hot. And, there's always the risk of hurricane season in September.
(4) From an activities perspective, my kids have really enjoyed MNSSHP. However, the downsides to MNSSHP are that it costs extra (and, it's not cheap), and that MK closes at 7 multiple nights of the week for that party. That gets annoying when you are not going to the party because it cuts into your time in the parks. In May, you can go to the MK every night if you want. And, because we stay at WL, we like to end up at MK when we can.