I must say being new to cruising, this only being my third, I was a little hesitant to try a different cruise line (Norwegian) but my husband won out. He always does! That being said, the Norwegian Sky was amazingly clean! I would give it a 4 star on that alone. Maybe it was because the Carnival ships are older, I don't know.
Embarkation was excellent. My brother and his fiancee came over from England and we had four from US (6 of us altogether)and we had no problems with passports or security. Excellent.
The rooms seemed to me to be a bit smaller on the Sky and I preferred the showers and room setup on the Victory, but here again - CLEAN!
I LOVED the dining arrangement on the Sky! We had no problem with seating. Tables were always available. The staff were excellent and, again - CLEAN!
The only thing I did find was that the menu choices seemed to be limited in comparison to Carnival. I preferred Carnival's food and of course the entertainment factor is missing on Sky.
The buffet area on the Sky, definitely superior to Carnival.
Entertainment: better on Carnival by far!
Disembarkation: a breeze.
All in all, it depends what you are looking for. Both cruise lines have different personalities. If you're the party hard type, go Carnival. If you're picky about a clean ship, go Norwegian. If you're big on the entertainment factor, go Carnival. If you're more into sitting on the balcony and kicking back, go Norwegian.
4 out of 5 stars though for sheer cleanliness!