The "better" destination depends on whether you prefer adventure or romance. Belize is the choice for adventurers and divers; it boasts the world's second-largest barrier reef and ancient Mayan ruins hidden in dense jungles. It has a more laid-back, "unplugged" vibe. St. Lucia, conversely, is the quintessential honeymoon destination, famous for its dramatic, lush Pitons (volcanic spires) and high-end luxury resorts with "three-wall" rooms. While Belize offers world-class snorkeling and cave tubing, St. Lucia offers breathtaking hiking, volcanic mud baths, and a more "polished" Caribbean aesthetic. If you want a "rugged" experience with a mix of Central American culture and Caribbean sea, go to Belize. If you want a dramatic, postcard-perfect landscape with sophisticated dining and infinity pools, St. Lucia is the winner.