EMH evening parks are definitely busier than EMH morning parks. We usually go in September and we've found that sometimes the EMH evenings are busy, sometimes they are not. If you can do the mornings over the evenings, then do so! You only get an hour in the morning, but there will be considerably less people around than would be during a 3-hour EMH evening.
The other thing to bear in mind is that since they stopped doing wrist bands for the evening EMH, it might look like there are more people in the park because they cannot see from just looking at you whether you should be there or not. I've not been asked for a room key or ID when purchasing something, which has changed, as you always used to have to show your wrist band in order to purchase something so that non-resort guests could not ride or buy, and therefore would be more encouraged to leave.
The only thing we had to show our room keys for this last trip during EMH was to ride the rides. And even then, in the same park, some were details looks, some were not. Buzz Lightyear's Space Rangers' CMs had torches and were looking very closely at keycards; Big Thunder Railroad Mountain were happy with just a flash, so those with old KTTW cards could probably got onto the ride, had they brought with them old cards!
