In my opinion, both are good value. There are some subtle differences between the two but they both target the same market and do a good job. I'd take either one, and if they are doing the same itinerary I'd take the less expensive one for the same cabin.
If you are unsure, why, take both of course then you can decide which one is best for you. <wink>
I agree with Thomas. RCCL feels more like a hotel inside, while Carnival goes for more theme/fantasy decor. I've sailed both to the Mexican Riviera several times. I think I would probably be rate-driven.