I would pick sometime towards the end of your date range, crowds lessen the closer you get to the end of August because of more schools and colleges starting back up.
For myself, I would look at the park hours. We stay on-site and always do the EMH, particularly the morning ones. I would want to make sure I was able to do morning EMH at MK and at least one other park. I would pick a 5-day span that satisfied that criteria. While, the August hours won't be set for a few months, the EMH periods listed will stay - they often add EMH, I don't know if I've ever seen them cancelled.
Are you flying? If so, the 3rd thing I would do is price flights - it's always more expensive to fly on days surrounding the weekend, than mid-week. So, the prices I could get on flights for different days would enter into my decision also.