We've been on 17 Carnival cruises and only 2 NCL. We just got home from a 4 day cruise on the Sky and loved that ship! It didn't seem as crowded as the Carnival ships we've been on, less chaotic on the pool deck, less announcements. On the down side, we didn't find the food nearly as good in the main dining rooms as on Carnival. NCL's breads and pastries are great!
Cabins on the Sky are smaller than Carnival's cabins. Service on both lines seems to be comparable.
I enoyed the Sky so much that I talked my husband into booking a B2B on the Sky in Nov.