knotnow is correct.
Are you sailing out of Rodney Bay or Marigot Bay?
A few days on a mooring
(you are not allowed to anchor) between the Piton's would be absolutely enchanting and could be enough before you take off to Martinique.
The Piton's are a world Heritage Site and the hotel
there is a great place to hang out and enjoy the scenery. Also if your desire is to see the countryside then there are day trips into the Jungle and you can climb the Pitons if you desire. There is a nice reef to dive on "around the corner" by a hotel
called Anse Chastanet.
Would not go any further south past the Pitons but would head
for Martinique making sure you stop by Marigot if you are not already sailing out of there.
If time runs out and you cannot make it to Martinique then spend a few days in Rodney Bay where you can anchor
, play on the beach and take a dingy into the Marina if you want that scene. It is an IGY marina with lots of shops restaurants, megayachts etc.
This will be a completely different experience than the BVI's not only in the sailing but also the topography.
It really depends on what you are looking for. Kept my boat in Rodney Bay for two years and always loved the stop at the Pitons before heading south to St. Vincient and the Grenadines.
Remember you have to clear out of St Lucia
to leave either going south or north. Both Rodney Bay and Marigot have customs/immigration offices. You will have to clear in and out of Martinique or St. Vincent/Grenadines prior to returning back to St. Lucia. No big deal on those particular islands but you HAVE to do it.
Enjoy your trip!