I have done both. I feel that there are more similarities than differences. My RCCL cruises were ten and fifteen years ago to the Carribean. MY Carnival cruise was three weeks ago to Canada. (enjoying a cruise on the first day of school the year following my retirement from 34 years of teaching.
I worried about this cruise because I thought that Carnival was going to be substandard compared to RCCL. (We chose it because of time and location.) The Victory was a beautiful, clean ship with wonderful shows, large well kept cabins, and the smoothest ride I've ever had.
My only complaint was that I felt the food should be hotter.
I did, however, really enjoy the Viking Crown Lounge on the RCCL ships.
Otherwise, you'll have fun regardless.