My wife and I were traveling with my 18 year old son and we picked Carnival because of its reputation as a party ship. It turned out to be a wise choice since we rarely saw our son during the 8 day trip. He made a lot of friends. I wouldn't suggest Carnival if you don't have children and are a couple just looking for relaxation.
This is my second cruise. Last year we went on Celebrity's "Zenith" and enjoyed it immensely. It wasn't as much of a party ship but my son, on that trip, had brought a friend along and the two of them stayed in a separate room.
Embarkation