We have cruised on Carnival, RCCL, NCL, Princess (before the Carnival Corporation acquisition), and a couple of small Mediterranean/Black Sea passenger accommodations. The cruise experience is a bit different between RCCL and Carnival, but we would agree that they are not dramatically different on whole. I would suggest that the following account for bigger differences in the cruise experience even on the SAME cruise line:
1 Cruising during the summer or spring break versus cruising during the off-peak periods. During the peak periods all ships are more fully loaded, lines are longer, and there are more children on any cruise line.
2 Cruising on different size and age ships. There are some highly rated older ships of Carnival and RCCL that match their newer ships for service and cleanliness; pick these or lower your expectations for a bargain cruise on an older or smaller ship.
3 Cruising on a ship immediately before a refresh versus after the refresh.
4 Cruising on different itineraries and from different ports. The same ship leaving from the East Coast, versus from Florida or San Juan, or the Eastern Caribbean, versus Western Caribbean, versus Mediterranean will yield a different experience.
5 Unfortunately, all or most cruise lines are reducing some of the perks as the passenger volume and costs increase. So comparing a cruise 2 weeks ago to a cruise on a similar ship and itinerary that you took 3 years ago will probably not provide an apples to apples comparison.
In summary, we enjoy both and look for the good deals. We believe any week on a highly rated cruise ship is a blessing to enjoy, even if not perfect, and that the differences are a positive that help keep cruising fresh and inviting. We do have a few RCCL cruises lined up, but that is mainly because right now RCCL has more ships we haven't cruised on compared to Carnival's top 3 ship classes by size. We may go back on an NCL cruise just because we haven't cruised with them in a while and we will probably pick a Celebrity ship soon. We expect and understand that the NCL and Celebrity experiences will be different than either RCCL or Carnival and that will be just fine with us:-)