you will find the two lines alike in many respects but different in some. NCL with freestyle cruising give you more dining options but you will not get that special bond that some like to develop with traditional dining. Cabins are a little smaller on NCL. Service is normally very good and the crew extremely friendly. Entertainment will be similar. We prefer NCL because of the casual dress, the friendly crew and the freestyle dining.
If you want to dress on formal optional nights, you will not be the only ones in formal or semi formal wear but it is nice not to have to take all the extra cloths just for 1 or 2 nights.
Dining, some will say Carnival has better food, others NCL> we have found them to be pretty much eqaul.