I've been on many Norwegian cruises as well as many Carnvinal cruises (my next Carnival will be my tenth) and I wouldn't keep booking with Norwegian if I didn't have just as great a time as I do on Carnival. Coincidentally, have the Spirit booked for this March (spring break week) out of New Orleans, but if you're looking for a bit more action, I say book Carnival.
They're known as the 'fun ship' for a reason. Just a bit more action going on than compared to other lines. Not to say you wouldn't have a great time with NCL or other lines, however.