Carnival and Royal Caribbean tend to have more energy and a younger crowd.  More children on these lines.  Celebrity tends to attract a more 'mature' crowd.     I would say the food in the main restaurants is a step up at Celebrity with the exception of the pizza/burger joints.  I give the nod to Carnival for the burgers and 'fast foods'.   The shows at Royal Caribbean tend to be a step above but often you have to make reservations.  Not so with Celebrity.