I think it depends on what your needs and expectations are. If price or itinerary is important, Carnival might be for you. If ship decor or activities are your thing, Royal Carrib., is for you.
Since I have been on both CCL and RCCL, I have to agree with Sonny re: RCCL, especially the Voyager class ships. That promenade rocks!! The ice shows were infact awesome as he stated. Food, I thought, was better on CCL. Cabin was definitely bigger on CCL as well. For my vote, I would say look at the Voyager class ships on RCCL. They truly are incredible.