The bottom line about EMH is that if you're staying at an on-site hotel, you should take advantage of the hours, but be willing to hop to another park when it gets crowded. Early EMH will draw on-site guests for the extra hour they can access the park before off-site guests. However, once that hour is up, everyone else will start flooding that park, and those who don't buy park hoppers are stuck in that park for the entire day. However, if you're an off-site guest, you should avoid the early EMH park because by the time you'll be allowed to enter, the lines will have already formed and FastPass return times will likely be after noon. All of the on-site guests who don't have park hoppers will be stuck in that park for the day, so it's usually better to go to a different park. Also parks with early EMH don't have an opening ceremony, so if you like seeing those, you might want to avoid the early EMH park.
Late EMH are a little different, not only because they're usually more than an hour, but are not quite as popular, especially on really busy days when EMH extends closing times to 1 AM or later. Late EMH are great for guests without families, but in my experience, they dont' affect crowd intensity during the day. So while you might not need to avoid a late EMH park if you're staying off-site, you might want to take advantage of the late EMH if you are staying on-site and want to marathon some rides. However, a good rule of thumb is that if you don't have park hoppers, you should avoid any EMH park so you lessen the chances of getting stuck in a park that's packed. If you have hoppers, you should probably avoid an early EMH park, but you at least have the flexibility of hopping to another park if you feel it's too crowded.
I find the MK EMH at night are the best. Nightime rides on Thunder Mountain, Haunted Mansion and Pirates? Yes please!
thank you for you advise