Unfortunately, the impact of after hours events don’t have the same impact as say the Halloween party at MK. People who buy a one day ticket to Epcot on a day with after hours still get a full day from 9-9, so I don’t see why the event would have much impact on crowds during regular hours. The EEH hours might though. Deluxe guests without hoppers will likely go there on that day perhaps increasing crowds throughout the day. If you’re a deluxe guest and you have hoppers, def go there for the EEH.
With all that said, go on the day that makes the most sense for you and your party. Develop a park plan and try to execute it as best you can (but also realize that nobody’s park plan goes exactly as they scheduled!).