I really don't have anything to add to Dave's comment except for the bit about departing before main seating commences holds true for Carnival also. But, a couple of times it was straight from the ship entrance to the room to change and then to the dinning room because of the timing. But it really is not that big of an issue. Certainly not one that would make or break my dinner seating time choice. We like the early seating because my wife can eat a big meal past 7pm without it disagreeing with her.
We have been scheduled a late seating on our Mariner cruise and the first thing we are planning on doing is seeing the maitre'd and asking to be moved. If we have to eat late, I'm going to be gaining a lot of weight because I will be eating at the buffet at 6PM "to hold me over" until 8Pm...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013