With main dining, you see the show after dinner. With early (main) seating you see the show before dinner. My family is made up of early birds. If we eat early dinner we end up being too tired to see the show. If we eat late dinner, we see the show first and then eat dinner. For some reason we have absolutely NO problems staying awake for dinner!
Here's pros/cons of early/late seating:
Early - you get to eat early enough to enjoy the dessert buffet. Entertainment is all after dinner, so that part is finished before you start seeing shows/entertainment. You get to eat pretty soon after returning from your shore excursions, so you don't have to snack much. You're always on time to special entertainment like the pirate show.
Late - there are less children that are misbehaving. The shows are early enough that you're not sleepy for them. You get to enjoy the snacking up on deck 9 a bit more. You don't have to rush back from shore excursions (or leave early) just to get ready for dinner.
HTH!
Sue