I've been on royal carribean to the bahamas... it was nice... the bahamas wasn't as nice as i expected it to be.
Royal carribean .. nice ship, nice dinners, nice casino.

Then we went on Norwegian to the Mediteranean. That was a trip of a lifetime. Italy and Spain are incredible.
Norwegian ships weren't "as fancy" but nicer with the free-style cruising. At the end of the trip the de-embarkation was really much easier on norwegian than the Royal Carribean cruise. The casino wasn't as nice on norwegian, but the ship was cleaner and the service was impeccable.

IMO...
If you're going with a bunch of friends.. go with carnival or royal carribean. If it's just you and a girlfriend/wife.. then go with Norwegian.