I just investigated this price comparison. Now I like the NCL option, too. You see a lot of Hawaii in the seven day cruise (in port every single day).
The longer cruises are for people who like days at sea - and believe or not the cost of a 17-day cruise is comparable to the 7-day NCL option after you figure in flights and the hotel you need to book if you want to see anything on Oahu (like Honolulu).
Figure about $4250 for two people for the NCL option (from L.A.) - while a 17-day mainland cruise is about $4000 oceanview (more for balcony).
But I like the actual Hawaii itinerary better with the NCL option - as long as you fly in early and get the hotel in Honolulu.