I can't tell you about food or service since I haven't sailed on Carnival, but based on pictures I have seen I can say that the ships' decor is very different. If you are more into glitz, neon, and the Hollywood-glam look, Carnival is for you. IMO, RCI is more refined, more soothing color schemes, more elegance. Carnival ships would make my eyes cross after awhile
The cabins on the Carnival ships that do the shorter cruises are usually a bit larger than RCI's, so if cabin space is important to you this should be a consideration. I think levels of service and food would be about the same. Carnvial also has a reputation as being a bit more rowdy than RCI, but that is going to depend on the people sailing a particular cruise.
I don't think you can go wrong either way. You will have a good time on either cruise, and there will be plenty for your son to do as well.