Eastern:
First two days at sea. One extra day at sea. Shopping (liquor, tobacco, jewelry, electronics, cameras, caribbean fashions). Beaches- topless and nude (St. Maarten), St. John- beautiful beaches, worth it to go and see the serene beauty, beautiful water, average eateries, little party life, nice and relaxing. Hills, greenery, lush landscapes.
Western:
First day at port, one less day at sea, Watersports, stingray city, gorgeous beaches, fun lively nightlife, eateries, bars and restaraunts. Shopping is very average unless you are looking for mexican tchazkies, tee-shirts, rum cake or silver. Best snorkeling in the caribbean (Grand Cayman) in my opinion. Flat, desserty islands, palm trees and gorgeous waters.
Hope that helps! (BTW, I prefer western, Hubby and I are in late 30's no kids).
Maureen