Again here is another opinion. I've cruised NCL 3 times, Royal 12 and Carnival 7. Royal is still my favorite, but we've been sailing more frequently on Carnival lately because of the price and itinerary. We like the mid to large size ships so for us we look at the Dream class on Carnival and Freedom/Mariner sub class on Royal. We're in our early 30's with a 4 year old boy.
We've typically had better and quicker service on Royal. Carnival seems to only have a few people making or serving drinks so we end up waiting forever for drinks on Carnival compared to Royal.
Kids about the same on both lines. We try to cruise off season if we can since our little guy isn't in school yet.
Royals balconies are so much bigger and deeper, rooms are appointed a lot nicer as well. Also Royal's suite benefits outweigh the non-existent carnival ones.
Food quality is about the same. The Royal AMPed ships have lots of new free casual dining finally catching up to Carnival. The buffet on Royal is nice and there are a variety of foods. Royal's specialty restaurants is where their food stands out. I used to give a slight edge to Carnival for casual dining with the Mongolian wok, tandoori but with the tandoori gone not so much. Selection of drinks and variety of drinks at Royal's bars are outstanding. I feel like on Carnival every bar menu is the same.
Production shows or Royal are awesome, Carnivals Comedy clubs are amazing. That's a tough one.
Ship layout, Carnival has the external promenade that wraps around the ship. Royal has the Royal promenade which I really like, lined with bars and stuff.
We're never had a bad vacation on any cruise we've taken and we've learned to diversify our cruising portfolio. We're looking forward to trying NCL again, and Holland America for the first time.