We have sailed 30+ times, we aren't particularly brand loyal. We have sailed Carnival, RCCL, NCL, Disney and Princess. However, most of our cruises have been on Royal Caribbean and Carnival. We love both Royal Caribbean and Carnival.
No Carnival ships hold a candle to the Oasis ships (or even the Voyager class ships imho). We are to the point where we are willing to pay up to 20% more to sail on a RCCL ship vs Carnival.
With that being said, we find the service is generally better on Carnival. We also find the MDR food slightly better on Carnival as well. But entertainment is much better on Royal Caribbean and any of the newer Royal Caribbean ships are MUCH better than even the newest Carnival ships.
We always have a great time regardless. We generally choose our cruise based on price and the specific ship. We don't really care about the itinerary, as the ship itself is our destination.