Everybody has their on priorities, so the better choice for you might not be the same as for someone else. I'm assuming that you're interested in a standard week-long Caribbean resort or cruise, though there are plenty of cruises to other parts of the world too and shorter trips available to the Bahamas and Mexico. My family tends to alternate types of vacations, so I've done plenty of both over the years.
Say you're interested mainly in SCUBA diving or shopping, a cruise might be a better choice because you'd get the variety from the cruise's itinerary. If you're a sit by the beach or pool and read/drink type, either type of vacation is great. A cruise would cater better to travelers who enjoy active excursions like ATV, zip-lining through the jungle, rafting, swimming with dolphins or stingrays, etc. A history buff would also probably enjoy seeing a variety of cities rather than simply being on one smaller island for a week. Staying in one place limits you to just the opportunities in the vicinity of your hotel, so it seems that you better *really* like what your hotel and the surrounding area offers. The ships themselves vary widely and the newer megaships have a stunning number of activities and amenities that would be hard to find at a resort (surfing machine, ice skating rink, climbing wall, bowling alley, comedians and musicians). The downside to cruising is that you're likely going to be surrounded by people, so if solitude is a high priority, you may be better off looking for a resort that promises specifically that. Some resorts also include alcoholic drinks in their all-inclusive rates; if you're a heavy drinker, make sure to factor in the cost of paying for drinks on a cruise.
The main thing that I like about cruising is that it is usually less expensive than the cost of a room and similar entertainment and food at a resort. Its also usually significantly less expensive to fly to Miami or just drive to a nearby port, instead of getting a flight to an island in the Caribbean. What's "better" is different for everybody, but modern cruise ships and their itineraries tend to have much more to offer a traveler than simply staying in one place.