Having sailed on 50 plus cruises in the past 20 years, I must say Carnival cannot compare to Celebrity. Celebrity, IMO has better food, service and the crowd is less 24 hour party. Yet you can still find plenty of activities. I find Carnival is typically a very young crowd while celebrity is more diverse. If you are having a bachelor party and want a wild celebration, Carnival your choice. If you are looking for a more relaxed vacation Celebrity is for you.
I also find that Royal newer ships are a great balance