I've never been to St. Lucia, but I have been to the Bahamas and I would recommend you try the Windsong Resort. Its a very nice Boutique Style Resort with great ammenities. You won't find crowds, or have to wait in line for a beach chair, there's always more than enough. There is also a Pool,Spa,Gym,Restaurant and SNUBA on-site. The reef is almost directly in front of the Resort(great for snorkling) and they also have bicycles at no charge.They are currently offering specials on room, check it out. http://search.iqrez.com/specials/WindsongResort/.

While you're in Turks & Caicos you can visit their former cotton plantation, Dine at Gourmet resturants on the gracebay strip (some are a bit expensive, others are pretty moderate in price), horseback riding in the day or evenings, do water sports and book one of the boat excursions(I love the excursions).There's also tourist shops, a bank and grocery shopping not too far away.

Hope you can make a decision from my post and congrats on your engagement.

p.s I think the Windsong is suppose to have a Havana Club opened by the time you're there. Sounds like fun!