We have done both, and I will ALWAYS do the morning hours! We love them! At the MK, we went in and did buzz x2, stitch, COP, TTA, indycars x2, and peter pan before the park "opened". The crowds didn't even start to build until around noon. At Epcot , we did all of the land and sea before we even started to see lines. And at AK, GO STRAIGHT TO THE SAFARI!!! There were 10x more animals out in the early morning, and lots were eating right near the road.
We have done evening hours, and to me it is NOT WORTH IT, you have to wait for a wristband... spend time in line to get it. The park is packed all day with people waiting for the EMH's. , and it doesn't really thin out till you late, and by then we are all to tired to care LOL.
Another plus, depending on what time of year, if you go to a park for morning EMH, and stay, lots of people will leave to go to the other park with evening EMH. So it is a win win.
I agree that it is lots better to go to the parks that don't have the evening EMH, you will get to do lots more there than at the emh.