A difficult one and similar to what we have been thinking recently.
The majority of us look for an ideal combination of low crowds and perfect sunny weather (not too hot) - unfortunatley neither of your dates offer you that.
The last 2 years we have been mid August, the crowds historically are fairly light as that is when the US schools (or certainly most of them) go back after the summer break.
Last year was busier, the
Disney Dining Plan (daft dining plan), meant whilst the parks were not packed they were certainly busier than they usually are.
This was a pain as you hope to counter the high temperatures with the knowledge that the parks will be quiet(ish).
For us, despite the summer holidays being the natural time to go, bearing in mind my wife is a teacher - we would never go in August again.
Julie and Joshua just didn't handle the excessive heat at all, most days it was 95 degrees and personally whilst I wasn't too bad it was far from comfortable touring the parks.
Sorry Alanda I am not really directly answering your question am I? If you can cope with the high temperatures maybe August would be best, but sometimes your own circumstances can affect your decision of when you go, they certainly do in ours.
Hence we are having a year off this year and going back Easter 2008, yes it will be busier but in theory the weather should be very pleasant, possibly ideal and it is that (in our circumstances) which has made our decision.
Josh is autistic, and whilst perfectly mobile he cannot walk long distances so we take a mini wheelchair with us for use when we tour the parks, the thought of pushing him around in August again would be just too much.