Many people feel these are in the same class, others disagree.
They are both great for kids and singles but I really think it comes down to ships. That being said I will offer the opinion that NCL is a step above all but the newest Carnival ships.
Carnival does have that party image too so if you are looking to have some fun, it is a good option.
These are both 4 star cruise lines.
I agree but the prices for singles is still too high.