I'm no expert, but I consider anything from early March through the end of May to be *someone's* Spring Break (whether college or lower, from around the country). So if I travel during those times, I'm expecting madness.
The AP discount on multi-day tickets is 15% off *gate* price, and you have to buy them there at the gate. The discounts buying them online isn't 15% from what I've been able to calculate, and neither is the discount buying them someplace like undercovertourist. Or so I've been able to find. So it does actually end up being the best deal (from what I've gathered, but I could be wrong, do the calculations!).
We stayed onsite in December and even though it was a slower time, the express pass that the hotels offer was *awesome*. We're going again in later September and I hear it's a slower time as well, but we don't care, we loved that special express pass (the EP you can buy only lets you ride each ride ONCE per day...the hotel version allows you to ride however many times you want to ride).