You get more money for the 8 hour party. I think that's the only difference. And since Maria and Simon give you time off after the wedding for your honeymoon, you can take your time with the party. So, I'd go for the 8 hr, for more bucks, at least. …
Did you go to So Chic Downtown and click on it? If you do, it should give you two options: the Summer Event or So Chic. Do the event and your next quest should come.
What do you mean? The event ends in that time. It's already going on. You better hurry. I'm one star away from getting the dress, literally. First event I've completed. Yay me. 😉