I have been on Carnival three times and are now booked on RCCL to give them a try. I have had a great time on Carnival but was a bit disappointed in the ship and service on my last cruise the Glory. I have also sailed the Fantasy and the Pride. My favorite ship is the Pride, the service, food and general moral of the crew was fantastic, it will be hard for anything to compare to that cruise, for me anyway.
My bestfriend has been on RCCL twice, Celebrity once and Carnival once.....her take is shows on Carnival are superior to RCCL and Celebrity, food is much better on Celebrity but approx. the same on RCCL. Service she says is better on RCCL but her only comparison is our cruise on the Glory and I even found the service to be below standards on the Glory. I had far superior service on the Fantasy and especially the Pride.
Nightlife....well after reading every review possible for RCCL and Carnival I would have to vote Carnival but I will make that final determination after my RCCL cruise.
I believe no matter what cruise you choose, you will have a great time. Look at me, we had the worst cruise on Carnival as far as service and food on the Glory but the Glory ended up being our best cruise ever and that is because of the people we met and hung out with!
Aslo, according to Maritime RCCL and Carnival are in the same category, Mainstream Cruises.
Good luck!