I love both. BVI is great for the easy sailing and convenience of all the facilities.

Grenadines is better for less crowds and more secluded anchorages. Greater variety between the islands as well. Plenty of places to anchor so you save $25-30 every night - which adds up to quite a lot over a week or two. Downsides of the Grenadines is hassle in some places from boatboys (not as bad as it used to be) and the greater distances between stops (of course that is an advantage to serious sailors)