There are fewer people there during EMH than at RD for a regular day. No matter when you go, park opening will be a mass migration to Pandora.
If you don't want to go to Pandora, you'll be fine at either EMH or regular day opening. If you want to do Pandora, be at the front of the EMH pack, and you'll be getting off around park opening time, leaving you an hour to do what you want with no-to-minimal waits elsewhere in the park. Come opening time, the off-site people will start a new wave of people to Pandora, and you'll still be okay elsewhere in the park. Later in the day, it might fill up more than on a non-EMH day, but if you have hoppers, that needn't affect you.