Stayed at the Sandals Grande on St. Lucia for my honeymoon! I haven't done a lot of tropical vacations to compare but we really enjoyed it. That island is pretty cool...clean, quiet, scenic, etc. You should be able to travel by bus to the other resorts which was fun to mix it up for dinner. We thought the Grande was probably the nicest of the 3 Sandals there but it is definitely more of a big resort type feel. You might be ok with that. One of the others (Halcyon I think) has some villa type places to stay, but it seemed to us the Grande had more to offer. Several restaurants and pools to choose from and they even had an X7 there if you wanted to take a quick wakeboarding run. The all-inclusive part is great as well...food and drinks out for you all day long. I'm not sure how rowdy you like your night life, but it seems pretty chill at most of the Sandals there. Well, I'm guessing you know that as one doesn't go to St. Lucia to party it up.
So...other things to think about. Definitely get away from the resort and go check out the city. I haven't been to see many other cultures so I thought it was kinda cool to go check that out. You obviously don't get any of that as the folks working on the resort aren't quite what you experience when you venture into the city. We took a little tour which I really enjoyed...got to hear about the island, see where chocolate originates, etc.
Another thing, and I'm not sure how much you want to spend, but do yourself a favor and check out Ladera. (www.ladera.com
) It makes Sandals look like a joke. We split time there and at Sandals, and Ladera was probably the most amazing place I've ever stayed. Open 4th wall looking down at the ocean, open shower, private restaurant, sleep looking at the ocean, and you can even have a pool in your room. It was absolutely incredible. I wouldn't go to St. Lucia again unless we could stay there. Check out the pictures of the rooms. It is expensive but completely worth it if you can afford it. I hope I didn't just cost you more money! You'll have to let me know what you thought if you end up staying there. http://www.ladera.com/tabid/389/default.aspx