We have been in both mid-July and mid-August and found August to be cooler and have less humidity than July. The parks closed about an hour or so earlier than July, except for Epcot, but it was really not that noticeable. We also found the crowds to be a little bit lower in August.
Average heat indexes that I remember:
July - 100 - 110
August - 90 - 100
I just found it to be a lot less opppressive and humid in August than in July.