I need to chime in on this topic.
I have only been on Carnival and RCCL so I can only really speak about them.
I think most of the lines are about the same with the food quality, I think the biggest difference between the two is the service.
Carnival has the dancing waiters, RCCL does not. RCCL has a better room service menu than Carnival.
One final thing.
I LOVE THE CHOCOLATE MELTING CAKE!!!!
I had to get that off my chest :-)