I've only sailed Carnival and RCI (and RCI only once), but Carnival's food was hands down better (a lot better) than RCI's.
In addition to having a slightly better presentation, Carnival's food just TASTED better.
At the buffet, Carnival had a slight edge - their grill is better, and the fish and chips wonderful. Carnival's pizza also beat out Sorento's IMHO.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04