Been on two eastern and two western caribbean cruises. After 12
DCL cruises, Would prefer more new itineraries (like the southern ones next year)
If you are looking at the Fantasy cruises, they are both 7 nights which is a nice length cruise. Here are my "highlights" from each cruise...
Eastern: Orient Beach on Sint Maarten (well its on the French side - so it's St. Martin) - very beautiful beach, the water, sand and facilities are very nice. And you just might see some nekid people

walking around
Eastern: St Johns (from St. Thomas) - snorkeling here is very good. Trunk Bay is one of the top beaches in the World
Western: Grand Cayman - Snorkeling (pick any of them) are very good, Also sting ray city
Western: Cozumel: Xplor park - AWESOME Zipline courses (2 - with 8-10 lines each), underground rafting (save for last), underground swimming, "off road jeep course" that is above and underground. Several good beaches with facilities on the island (where the ship docks)
Western: Costa Maya - Mayan pyramids - very interesting excursion.
If you are doing the western on one of the classic ships, you might stop at Key West - we liked the Old Town Trolley (hop on/off), Truman's Little White House, Hemmingway Home, Southernmost point, Key Lime, Chocolate Dipped Key Lime Pies, Easily walk-able city area