I have cruised on both and each has it's good points. Carnival is probably still my favorite but there are definitely some things that NCL does better.
I would like to try a few different NCL ships before saying one cruiseline is better than the other. From my limited experience the NCL Sky ranked pretty low (think Carnival Holiday with bad food) and the NCL Jewel was one of the best cruises we have ever taken. One thing that was very noticeable both times was debark on NCL seemed much more orderly and civil. Did not see all the people jamming the halls pushing and shoving like on Carnival and you are allowed to remain in the cabins until debark. The pool music was more background and not abusive as it sometimes gets on Carnival. The cold towels they pass out are not bad either. Entertainment on the Sky seemed about the same as Carnival but on the Jewel it was better. Second City comedy was great and the Cirque Bijou show was as good as I have ever seen on a ship anywhere. Dress seemed a little more upscale at night on NCL but I am sure that will vary by cruise. JMHO 8)
56 Carnival: Mardi Gras, Ecstasy X2, Fascination X6, Celebration, Elation X5, Glory X5, Fantasy X4, Valor, Holiday X3, Imagination, Liberty, Spirit X2, Freedom, Destiny X2, Splendor X2, Dream X2, Conquest, Legend X3, Victory, Sensation, Paradise, Inspiration, Magic X2, Triumph, Miracle X2, Breeze, Sunshine X3
7 Norwegian: Sky, Jewel, Dawn, Epic, Pearl, Breakaway, Getaway
7 Windjammer: S/V Legacy X3, S/V Polynesia, S/V Caledonia, S/V Diamant, S/V Sagitta