hmmmm. We have been both times. let's talk it out......
Yes Spring break is packed. Yes August is hot. The good news is that the parks are open late during both times, so you can take a break mid-day and miss the worst of the crowds and heat, both times.
I think my important question is, where are you comming from? Gotta fly or will you drive?
I would only go during spring break again, is we were driving. It was the actual traveling that made spring break bad. The airlines are all over booked, so If you actually get a seat on your flight, the employees are as likely to spit on you as smile, because there are 20 people waiting for your seat, so if you get mad at them and don't fly, they really don't care. Everyone is frazzled, too. So traveling over spring break will be way more stressful.
Because of that, I would reccommend August. That's when we would go again. It will take planning and extra gear, like misty fans and such, but that's when I would go. Plus you are way more likely to get airline deals and hotel discounts in August.