Setting my last experience with Carnival aside, in my opinion RCCL has better ships, the entertainment factor seems to be higher / more interesting and the experience seemed to be better.
Although the price is higher, the old adage; you get what you pay for seems to ring true. I've cruised on the Voyager Class Ships on RCCL and the Conquest Class Ships with CCL and although the Voyager class ships of RCCL are older the gross tonnage and capacity is still larger than that of the Dream Class ships CCL is producing now.
In my opinion it seems like RCCL hired an interior designer and CCL had a honey-do list. I've never felt crowded on an RCCL ship the way I do on Carnivals ships and while on RCCL's ships I actually feel like I'm at a floating resort.
I chose a few CCL cruises because of the price but I don't think I'll be doing that ever again. I will save the extra hundred or so bucks it costs and just cruise RCCL.
Ports of call sometimes come into play when cruising but I can honestly say that I can see myself taking a cruise in the future and not stepping off the ship once. I feel like that's something I can do (not get off the ship) on RCCL, but not on CCL. I couldn't wait to get off the ship during my last cruise with CCL and dreaded getting back on when my day in port was done.
RCCL's solarium's and promenades out-do any social gathering areas that Carnival has yet to produce on any ship that I've been on. I also find it helpful that there is one folder where all your pictures are placed every night so you don't have to play hide and seek to find your photos.
I've heard it said that CCL has fun ships and RCCL has fun ships - with class! I tend to agree. Although CCL used to be my cruise line of choice, after my last experience, it's not fact anymore.