I've stayed at both La Toc and Sandals Grande and prefer the Grande. Unlike Halcyon and La Toc, the dive operation is on-site and you board from the resorts dock versus being bussed into town.
The Grande is at the North end of the island, but you dive the same sites regardless. The boat ride is a little longer Grande, but total transit time was longer at La Toc, including the bus transfer to and from town.
We we have stayed at their resorts multiple times and overall like what they offer - Great service, fun atmosphere, good food, ... They do include diving, but as some others have mentioned, it is limited to two dives per day (plus a couple night dives a week for an extra charge). Not bad if you want to start your day with a couple of dives, grab lunch and then explore the island, take part in other watersports,play some beach or pool volleyball, or simply chill at the swim up bar.
St Lucia is a beautiful island and there are a lot of topside tours that you can do during your surface intervals. For me Sandals gives a pretty good balance of diving and relaxing.