We've been on both many times and always prefer RCCL over Carnival no matter the ship we've gone on. But that's us. Keep in mind that what one person loves, another will hate.
This goes for the food, too. Some people prefer McDonalds, while others prefer Burger King, yet others like both, and others don't like either. Food is very subjective and everyone's taste is different.
But why only thing about RCCL and Carnival? Why not consider some other cruise lines?
Often you can get better without spending more money. So you have to ask yourself what's more important to you; money, food, quality, ship, itinerary, etc?
Talk with your agent (don't book directly with the cruise line or online) and have them present several options to you. You might be surprised as to what you may find. But it's important for you to find the right cruise for you based on your requirements, budget, and lifestyle.