I think Sandal Grand St.Lucia is much better, especially if one goes Scuba Diving. Eventhough you cannot do shore dive from Sandals, but their boats are big and fast, well-equipped and comfortable.
The coral reef at St. Lucia is still very "untouched" - although the best diving sites are just at the shadow of the Grand and Petit Pitons, at the foot of Anse Chastanet, the operator they used, Scuba St.Lucia, does not have nearly as nich equipments as Sandals.
I did a 2-tank dive with Scuba St.Lucia last Dec during a port call of our cruise. I wasn't impressed by their equipments. My regular leaked, and the diving master agreed. However, instead of changing equipment, she told me to use the Octopus (the 2nd, spare regulator on the side) instead!