I've never sailed on Carnival but allow me to offer my sisters perspective. Her and her boyfriend took their first cruise on the Carnival Magic. They came home raving about how great it was. They then sailed with us on NCL Dawn last January and have since said they much prefer NCL. Now that they experienced NCL, they felt the Carnival experience wasn't as good. I sailed on Royal Caribbean before NCL and I gotta say, NCL doesn't even come close to the experience and service of RC. So I would put NCL above Carnival but below RC based on these experiences. Again, I have never sailed on Carnival but definitely felt like a downgrade going from RC to NCL, where my sister felt NCL was an upgrade over Carnival.
Pete