Ok.. here's one man's opinion.
RCI has be far the more beautiful ships in interior and decors. Carnival has by far the better food, and food options... along with a wider variety of buffet items, particularly lunch items.
RCI has the ice skating, flow riders, etc. Carnival generally always has some fun and silly pool games etc.
Carnival takes their "Fun Ship" slogan seriously, with lots going on and a very busy and active night life. RCI on occassion has a an active night life.
Service staff on RCI might be a bit more "polished", Carnival staff are normally fun and friendly.
Finally.. if Carnival dates work better, I wouldn't hesitate to make the switch. At this point in time I prefer Carnival to RCI myself.