Michelle, peninsularoad is right about the waiters. If you see the waiters as entertainers more than fooddeliverers Carnival is probably better for you. My opinion is that the waiters are there to serve the food and then it's good that they don't sing and dance. Only you can say what you prefer! If you need the waiters to know you, that's easier on Carnival but you can ask to get the same waiters every night at NCL too. In my opinion it's not important that they know me and the service is also better then because they serve food instead of talk!
If you cruise to meet new people fixed tables can be better if you only want to meet people at dinner, if you meet people somewhere else on the ship you can't have dinner with them in the maindining room if you have a fixed table and time.