If you happen to visit during the slow months, I'd take advantage of it. We did once this past Jan, (evening EMH at MGM) and it was really alot of fun.
But if you go during the busy times AND you do not have the hopper option....skip it. You will be stuck in a very crowded park with no way out.
If you DO have the hopper option, I would say go for the morning EMH. People like to sleep in when they are on vacation so you have more chances of getting more done than during the evening EMH when the parks are so much more crowded.
If you do AM emh, you can hop to another park once this one gets too crowded.