It refers only to the 7-day cruises. The Magic sails alternating eastern and western Caribbean itineraries. The eastern has stops at St. Maarten, Netherlands Antilles and St. Thomas, USVI; the western calls at Key West, Grand Cayman, and Cozumel, Mexico. Both call at
Castaway Cay on the final day of the cruise.
As to which is preferred, that's a very subjective thing. I've been on both and enjoy them both immensely. If I had to choose a favorite, it would probably be the eastern for several reasons: an extra day at sea; no port call on the first full day of the cruise; St. Thomas (actually St. John, which you can visit from St. Thomas).
Some of the plusses for the Western, in my opinion are Grand Cayman and a bit longer stay at Castaway Cay (maybe an hour or two). I'm not a huge fan of Cozumel, but I do like that the ship stays in port pretty late there. It's fun to check out a little of the nightlife after dinner.
As I said, I've done both, and will do both again.