I've sailed both Carnival and RCI a number of times. Ultimately you aren't likely to wrong on either.
RCI has the more beautiful ships imho. All their new ships have a wow factor that Carnival's don't. Both offer lively cruise experiences.
I do think that over the last several years RCI has neglected their food dept. while directing their attention to the ship's asthetics.... and Carnival has vastly improved their food quality in the same time frame.
I'm sure you'll find opinions vary and as I said... you won't go wrong with either.