I have been 6 NCL cruises with our 7th planned and only 2 Carnival. We were on the Holiday in 1990 and broke down and tried the Miracle this past August. I was so pleasantly surprised with our cruise as it exceeded all of my expectations. We are looking into other Carnival cruises in the future.
I think that the biggest difference for me were the set dining times. We adjusted to this well and it was better than I thought it would be in terms of quality and service. We loved the buffet on Carnival much more than NCL. I thought that the portions were bigger on Carnival. I loved the supper club. NCL has some great speciality restaurants as well and we have been to all of them.
I actually think that there are more similarities than differences. Enjoy your cruise what ever line you decide to go on.