I had the very same dilemma for next year.
I've only ever been in September before, which is lovely and warm, and very quiet, but you do tend to have more afternoon thunderstorms.
But new life now (divorced from DH I went with before, and new DB now who has 2 sons, who'll be 12 and 8 when we go).
SO.... I want to go back, and take the 3 of them, but it HAS to be school holidays, neither the senior nor junior school will tolerate taking them out of term. The choice then was Easter holls (they get 2 weeks) or summer (they get 6-7 weeks, but earlier than the English schools, they go back to school right in the middle of August).
After much thought, we eventually plumped for Easter! The parks will be busy whether we go Easter or Summer, but I think both the boys would really struggle with the heat and humidity in the summer, which makes that a no no. Easter 2010 is the first weekend of the 2 week school holls, so most of it will be spent travelling, and settling in, and we'll leave the parks till after Easter Monday and hopefully it will be a little quieter then. We also plan to get up early and attack on park days, and go with a thorough plan to try to avoid the worst of the crowds.
I must admit, I'm really worried that the crowds are going to spoil what should be a magical holiday, and always has been for me in the past, but if I go expecting the worst, then if it's better, then it's a bonus! I don't have any choice to go at a quieter time now, as I wouldn't leave the boys behind and just go with DB. That wouldn't be fair.
Hope my ramblings help with your decision. It's a tough one, I know!