These type questions are really subjective. You can't really form an opinion without experiencing both lines. Just reading what everyone else thinks is not very valuable when the question is "compare X cruise line to Y".
With the disclaimer out of the way....I like both RCCL and Carnival. RCCL has more interesting ships with more "things" to do. Carnival has better buffet choices. RCCL has nicer dining rooms. Dining room food between the two is a toss-up to me. I like both. RCCL has nicer cabins overall, although Carnival has larger standard cabins. RCCL has the clear advantage with suites. Service is equal. RCCL has much better perks for repeat passengers. I think RCCL handles embarkation and debarkation better than Carnival. I think RCCL has better and more interesting bars. Carnival has glitzier big shows, but RCCL is closing that gap. Both lines are equal on the smaller shows and bands. Carnival intices me with some very good blues and jazz bands on their ships. RCCL seems to do better with the poolside entertainment. The casinos on both lines suck.
And there you have my 60 second comparison!