I agree with mom2rtk. In october, any day there isn't a party will be more crowded and an emh night will be even a little more crowded. Now if there is 2 days in a row without a party, the day after the emh day is usually pretty nice.
Epcot evening emh is strange. Most leave, most of world showcase is closed and there isn't that many rides but they seem to hold a line the entire time. It's also a pretty good chance the beer muscles are going to come out for mission space so it might be an interesting crowd on teh bus ride home (and smell) from them daring each other to ride it after a day of drinking.