I have been on both RCI and Carnival. I personally like RCI better due to the "White Glove" Experience. There are differences between the two but would never suggest not to sail on Carnival because I like RCI. I just had some things I like better on RCI:
1. My DH and I are not drinkers and RCI did not shove it in our face all day.
2. We do not party hard and Carnival is major partying. Not to say that parties are bad.
3. I enjoyed the dining experience better on RCI. The waiters were more attentive. This could be due to the fact we were at a 8 person table and only 3 of us showed up every night.