We have tended to take advantage of morning EMH more so than evening EMH, as you can gain alot more ground in that one hour before most of the guests arrive, than you can in 3 hours with the park still packed. Let's face it, even when it's only open to resort guests, when you think of how many resorts there are and how many rooms there are amongst all those resorts, and the amount of people that would constitute just HALF of that amount, that can make the park just as packed as when it's open to the public. It may be very cool walking around in the park for 3 hours after park closing just to see everything all lit up, but you may not have much luck getting on the major attractions.
When we have gone to Magic Kingdom, Hollywood Studios and Epcot for morning EMH at those parks, we were able to cover alot of ground in that one hour. So far, Epcot is the only park where we have done evening EMH and we really enjoyed it because we were with friends of ours, just leisurely walking around the parks and doing the minor attractions with no problem (SSE, Maelstrom, Grand Fiesta, LWTL, etc), but we didn't even attempt to do Test Track. I think we were able to pull of Soarin' one time just because we happened to be there before all the people migrated over, but we still had to wait a half hour or so in line.
So if it's the minor attractions you're interested in, or just seeing the park at night, then evening EMH is for you. If you're really wanting to hit alot of rides, including the major ones, then morning EMH would be the better bet.