Seeing you are posting on the Carnival section of the boards I assume you are talking about Carnival ships. They have two dining rooms and 4 dinning times. Early seatings are 5:45 and 6:15 I believe and late seating is 8:00 and 8:30 I believe. At least the times will be within 15 minutes of that.
The early seatings will have more families and kids. Some people think the early seatings make them rush to much to return from tours. The early seatings leave your evenings open for shows and the lounges. You would have to change clothing early for early seating also.
Late seating you can linger in the dining room a little longer because they do not have to get ready for the next dinning time. Late seating your shows will generally be before you eat in many cases a few shows afterwards. If you like the midnight buffets you may still be full from late seating dinner. If you like to turn in early you would be turning in on a full stomach. The bars and lounges will be open after late seating.
I find that I try to pick the dinning time that comes closest to the way I eat at home. We always have early seating and only twice did we have any problems returning from a tour for dinner and that was because the tour bus broke down.
we ate at 8:15 and it seemed perfect..besides not having to rush back from tours or the pool to shower and change clothes, we were always still 'full' at 5 or six o'clock from lunch or snacking! you will have plenty of time to go to the show in the evening which starts at or around 10:30