Really, it depends on the park. For instance, I find that the Magic Kingdom is going to be crowded at any time, but generally, with the extra magic hours, there are more people in the early am hours rather than the later pm hours (cause the little ones will go to bed early). But, Epcot or MGM, early or late, doesnt matter, cause during the extra magic hours, there seems to be virtually no wait for any attraction. Of course, the major draw back to the later extra magic hours is that, when you leave the park, you are leaving with everyone else staying at a Disney Resort, which means you'll be waiting a long time for a bus.
But, in short, generally whether is morning or night, any time during extra magic hours is a good time to be in the park, cause its generally never that busy.
hope that helps,
BT