The VI's are amazing. I was sailing in the BVI's last year and had a great time. I've never actually been to the USVI's though. They looked to be quite a bit more built up than the BVI's. I've sailed a bit in the southern West Indies as well but the BVI's have been my best experience down that way.
My parents actually spend 3 months out of the winter every year sailing around the BVI's. Their actually there right now. Wish I was.