As someone else suggested, bring a little bit of everything. Things like gloves and hats (and warm socks!) don't take up much room in your luggage, but can be expensive to buy if you need them for your whole group.
Here's my experience: I went October 8-18 last year. We had record-high heat for the first part of the trip, and record-cold temps for the last few days we were there. What I wouldn't have given for a warm jacket, knee socks, and gloves! I had packed "correctly" for the first week (shorts, sleeveless tops, sandals) but had no idea the temps would drop so drastically later in the trip. It was so breezy and cold, even though I was wearing long pants, the cold wind made my legs freeze (I only had short "ankle" socks). I was at Epcot, and finally found a pair of warm socks at the Japan Pavilion after searching for more than an hour. And remember, this was mid-October, not December!
So I say: bring a little bit of everything; be prepared to layer; bring gloves, hats, and warm socks for everyone; and have fun!