Like Oceanlover sez,,,there are good and bad points on both lines. We've done both extensively, and really like both,,,except for the RCI megaships. I simply like the smaller RCI ships much better. I doubt that you will find anything really wrong with either line, and suggest that you book which ever line offers the iten, and the price that suit you. The perks for either line aren't that great until you get to the Platinum, so IMO that shouldn't enter into the decision.
Oh yes,,,the ice cream/frozen yogurt machines are open 24X7 on Carnival!