I took my first cruise on RCCL (Majesty), my second on Carnival (Fantasy) and every single cruise after was with RCCL. I agree that you have to experience each before you can make a decision on what is better; it really depends on what is most important to you. Now mind you, it has been a few years since I've cruised with Carnival but if I remember correctly, my room was bigger and it had a nice little pretend window, and there were alot of dance clubs. That's all that I remember being more impressive... oh and they had a nice little Silver Jewelry Shop. But I spend very little time in the room so that did not keep me with Carnival. Other than that... I remember it being very unorganized- I am super OCD and if it had been my first cruise, I would have gone insane! I never got the daily forms that tell you what activities are going on and disembarkation was a disaster... The ship had an awful odor from serious sea sickness in the prior days... and contrary to what people say, I still found that it was a constant drunken party boat. I'm young and can enjoy a good party, but it got a little out of hand with people so drunk they were sliding down banisters and across counters. Another deterrent for me were the colors of the ship. This is probably not important to most people, but it is for me. When I'm on vacation, I want it to be relaxing. For this same reason, I don't paint my bedroom bright red- it's not very peaceful. Carnival is based on Crayon Colors.. Red, Dark Blue, Green, Bright Yellow.. etc. RCCL is very pastel-like. It's soothing.
So.. it's obvious that I am a fan of RCCL more than Carnival, but it could just be the one ship that I was on, and it could just be the time of year, or the specific stateroom attendant I had... I have never cruised with Carnival again, but obviously there are people with great experiences, or they wouldn't be in business. I hope you try out multiple ships and get your own taste of each. Happy Sailings!