There is a strong rumor going around that the Moorings is moving it's base to Rodney Bay. If so this makes the Martinique itinerary very easy. Regardless, you'll likely depart Rodney Bay for Martinique; Cul De Sac Marin is a large and well protected bay. Le Marin is a good size town with excellent provisioning in the Supermarchés, if you like French wines and delicacies; it will shame the Moorings provisioning. Personally I would only provision what I need to get out of St Lucia and do real provisioning in Martinique.

Lots of interesting things to see and do on Martinique, it is a full Department of France (like Hawaii is a US state) so you get a real blend of metropolitan France with a full carib overlay.

Diving is very good by Carib standards with a number of wrecks and interesting sites, Dominica is likewise very good diving.

Have you considered charting from Martinique? or my favorite Guadeloupe. Reasonable sail to Dominica from either, but you get the Ile Des Saintes from Guadeluope. DYC will let you do one ways (for a fee) between them. In January the prevailings will be north of east, so a sail down island from Guadeloupe might be easier on the crew.