Hawaiian cruises are normally 11 nights, departing from either Ensenada, Mexico or Vancouver with 5 days at sea before arriving at the first Hawaiian island. Not much time in any one island (except an overnight in Maui), so it's a good way to "sample" the Hawaiian islands. I sailed the Legend from Ensenada in March, and although it was a great cruise overall, the weather was terrible (winter in Hawaii is rainy season!) And keep in mind the flying, especially if you are on the east coast. And also keep in mind that this cruise is much more expensive than a 7 night Caribbean and a MUCH older crowd, almost exclusively retirees.
Having done Hawaii and the Caribbean (eastern, western and southern), I would prefer the Caribbean. Each itinerary (east, west, south) has it's own high points. Western Caribbean I love Grand Cayman, but don't like Ocho Rios. Southern, I love Aruba, but you have to fly to San Juan (vs. perhaps driving to Florida). Eastern, I love the shopping and the islands of St. Thomas and St. Maarten.
If I absolutely had to choose, I would pick the Eastern Caribbean!
But of course, ANY cruise is wonderful!!!!!