In all fairness, I haven't sailed Carnival in about 5 years or more, but I have many clients that have. The reviews are mixed, pro, con and inbetween.
Carnival has larger cabins, a little more active night life and the traditional dining which appeals to many. The biggest complaint I have heard is food quality and service, but again food is too subjetive. We found the food to be ok, not great but oK. WE did like the lunch variety in the buffet.
NCL has the better dining choices, a little better service I think and better shows, again all subjective. Most mass marketed lines are more alike than different.
The biggest complaint I had with Carnival was how pushy they were in the spa. I had a facial and I thought i would have to play dead to keep the gal from trying to sell me every product they had, not to mention insulting me and my skin.
My skin acutally cried because it was so sad...
I think you can enjoy either line.