Having now done both, I find both lines to be completely interchangeable, similar, yet different. Both have their stong points. Both have their weak points.
My suggestion is to get out the and plan for BOTH! You just have to figure out which one you want to do first!
If for the ship, I'd chose RCCL.
If for the accomodations, I'd go Carnival.
And for the "experience", I'd go Carnival as well. The only other line that has the "experience" is Disney.