We are in the Celebrity program - they changed it after we joined in the 90's and we were essentially grandfathered into the new program which means we get priority embarkation at our member level while new members at our level don't get priority embark. Of course, being platinum in the RCI program probably means more for us if we cruise with Celebrity these days.
Carnival doesn't really have a loyalty program worth noting, at least in my view, and we've cruised extensively with Carnival over the past few years.
For my money, RCI has the best program. But as you know it doesn't really kick in until you get into the platinum level and then it really takes off when you reach diamond and diamond plus.