We have sailed both several times and each line has attributes we enjoy. As others have said, the activities on Royal seem more upbeat and lively while Princess is more subdued and relaxing. The food is similarly on par with each other. We've sailed on RCL Vision, Radiance, Voyager, and Freedom class ships. Of those, we liked the Rhapsody and Liberty the best. On Princess we have sailed R, Coral, Grand and Royal classes. Our favorites, hands down, are the Grand class ships. The layouts, aft pools, and Caribe extended balconies are a big plus for us. We're looking forward to our upcoming cruise on the Caribbean Princess. While we like the Pacific Princess, it felt a little small. I hope, someday, Princess will introduce a class ship similar to Viking with a passenger count of around 930-1,000. We're not fans of mega-ships unless it's advantageous to do a TA on one.