We have done both these intineraries several times on the Explorer of the Seas and the Carnival Miracle. We will be repeating the Grand Turk, Half Moon Cay, Nassau cruise on the Carnival Splendor this coming August. Both are GREAT cruises, but we like Carnival a bit better. A couple of opinions...
The Explorer is a much nicer ship, with a lot more amenities, which translates into higher fares. If rock climbing walls, ice skating rinks, in line skating, etc. don't mean that much to you, go for the balcony cabin on the Splendor.
Comparing the food on both lines (dining room, lido, etc), Is a very subjective subject, but we feel that Carnival is better....Hands down.
Itinerary wise, the Explorers Bermuda and Labadee, though very nice, are trumped by the Splendors Grand Turk and Half Moon Cay. The other ports on both lines, San Juan, Nassau, and St. Maarten we have been to so many times, that they are not a factor in selecting an itinerary for us.
Again, these are just our opinions. Have a GREAT cruise whichever line you finally cruise!!