Depends what you like. I love the western. I have never been to Key West, but it's supposed to be great. (I'll let you know after I return from my trip next week!!!!!!) I think my favorite island to visit in all the Caribbean are Grand Cayman,(along with Aruba, Barbados and Grenada.) Cozumel also gives you a taste of Mexico. On the Eastern (If we are talking only Disney Itineraries now), The islands are great, but there is not much difference in St. Thomas and St. Maarten. Both Hilly, shopping, good nice beaches (comparatively). Very similar. So, depends on what you like. I like the variety, the water and beaches, and the activities on Western better.
Some will argue that St. John is beautiful, and it definitely is, ABSOLUTELY BEAUTIFUL, but again similar to Sts. Thomas and Maarten in culture and scenery. Some will also discuss the extra day at sea on eastern as being a plus. I prefer the extra port. Both go to CC so there is no argument there.
Mrsfuzzmo