After doing the BVIs several times, we did a St. Lucia to Grenada one way through the Grenadines last year. I highly recommend it. We had a sailing cat, not power, so you will have more flexibility - starting and ending in Grenada or St. Lucia depending upon the best travel and boat choices.

There are several great places to explore, Bequia, Tobago Keys, Salt Whistle Bay, Union Island, and more. It is not as developed or as touristy as the BVI - which I think is great, but others may miss some of the modern conveniences of the BVI. Restaurants, for example, are definitely more 'rustic' - your menu choices are in most cases limited to chicken, fish or lambi (conch) with rice and vegetables. Of course, you can always go to Mustique for some finer dining. The trade off is relatively fewer other boats, though some anchorages, like the Tobago Keys will still have plenty of other boats. Depending upon season, you should have no trouble finding anchorages with 1 or 2 other boats if any. Moorings are sometimes available, but much more limited - you will anchor more.

You may also want to look into the Exumas. We went this past November and loved it. Again, more remote than the BVI, and although the restaurants are few and far between, they are far more upscale dining experiences available (than the Grenadines) due to the frequent high end clientele around the Exumas.

Like you said, you can't lose.