We this past June, the 3rd-11th. It was our first summer trip. All in all, it wasn't as bad as we expected.
The crowds were heavy, but managable. By getting to the parks before opening, having a carefully thought out plan, and making good use of Fast Pass, we didn't really have any crowd issues to speak of. (I'd seriously reccomend Tour Guide Mike if you go in June or at Christmas!)
The heat was pretty horrible, but we tried to be out early and then head back to the resort to nap/swim after lunch. I'm used to hot weather in the summer, but I still found that combination of intense sun and horrible humidity to be absolutely BRUTAL. We probably won't ever go in the summer again for this reason.
We do love the parks at Christmas because of all the festive atmosphere, special things going on, cooler weather, etc. However, we usually go in early December when things are very, very slow. I've heard that the week between Christmas and New Year's is absolute gridlock!
So, I guess my answer is . . . . I'm not sure which would be better. Sorry for not being more help!