We were at WDW June 29 - July 5 and did EMH for all parks except for Epcot. I had very good results with EMH. We were able to do everything we had planned before noon. We were even able to get a few fastpasses during the EMH, which meant we were able to do a few rides twice. My advice is that if you do plan on doing EMH, plan on leaving the park between 12:00 and 1:00. By that time, the park is extremely crowded.
On MK EMH, we only did Fantasyland and Adventureland. We had done Tomorrowland and Frontierland during E-ride night. That meant on MK EMH day, we skipped those and did the things we hadn't done yet.
Like another poster said, do it both ways and see which way you prefer. Just remember, if you plan on using WDW transportation, be at the bus stop 1 hour before park opening on EMH day (7:00) to make sure you get to the park to enjoy the extra hour.