There are a few considerations here.
1. I'll assume you are staying in a Disney resort and thus eligible for EMH. If not, then the general consensus is to avoid the EMH parks.
2. If you don't have park-hopper tickets and want to spend all day in the same park, it's generally considered a good idea to choose a park without EMH. For most of the day, a park on an EMH day is almost always more crowded (and has longer lines) than the same park on a non-EMH day.
3. Many people with park-hopper tickets like to go to EMH morning parks until it starts to get crowded (maybe until lunch time?), then hop to a different park in the afternoon.
4. A lot of people also like to choose the EMH morning park even without park-hoppers, because they plan to go back to their resort for a rest or a swim in the afternoon. Then maybe go out for an ADR or fireworks show in the evening.
5. Many people love EMH evenings, as well. But they usually recommend it only if you are willing to stay until closing. It seems to be the last hour or two when the park starts to empty out and the lines may become walk-ons for many rides.