Only one park per day has the Early Magic Hour (EMH), and only a certain number of rides are open then. For example, MGM has Star Tours, RRC, and ToT open - but you can't see the Indiana Jones Show. Magic Kingdom has most of Fantasyland, but also Space Mountian, BuzzLight Year, Splash Mountain and Big Thunder. So when you do EMH here you have to decide which part of the park you want to go to so you can really enjoy that first hour. You're not going to be able to do all the rides in MK without running and stressing out. MK is EMH two mornings a week (Thur. and Sun.?) so do both.
We don't typically do EMH at Epcot, but now with Mission Space, we may decide to try it!