kymberly,
If you are planning to use a day's admission that day anyway, the MK is probably a good place to be. But if you'd rather save a day's admission (sleep in, go to a water park, etc.,) that day, then you can get into the MK just before the party. (And I think after 4:00 if you have PS for dinner).
Since MNSSHP is a separate ticket event, you wouldn't have to be in the park that day to enjoy the party.
Personally, we're making it a leisure kind of day so we can last until midnight!!