Amanda I just emailed you, but it is so worth it to stay on site at US/IOA because of the front of the line access. Even if you can get 1 night in at a US/IOA resort, that will get you 2 full days of FOTL access. Then, crowds would not be a problem. We took the kids in June of 2003, and we able to do everything we wanted a number of times because of FOTL access.
EArly september to mid september is wonderful..It is still hot but the crowds are gone after Labor Day. Even Mummy was a walk on in September all while we were there....go and enjoy!!.....
We are annual passholders and love Universal and IOA. Even with our discounts the resorts are too expensive. We've done it twice for a couple of nights and vowed to never spend that money again in Sept. We go every Sept, and both parks are dead. There is absolutely no need to stay on site in Sept. The other poster said that he/she stayed in June. I agree if you go during a peak time you'll need the FOTL but not for Sept. Just my opinion.