The last week of June will not be as hot as the last week of August, but it will be more crowded than August. Also, you can look at whether you want your trip at the beginning of the kids' vacation or at the end just before school starts back. I've done both and the only drawback to going at the end of summer vacation is I have to go back to teaching tired from my vacation.LOL The good thing about it is I have going to WDW to look forward to all summer long! Either way, you will have a great time.
For the temps alone, I would pick June. We've been to Disney during both times, and the weather in June was cooler and less rainy. In August, we pretty much spent alot of the time trying to find shade! And those late-afternoon thunderstorms happened every day that we were there.