I haven't done both but having done two charters that start in the BVIs I don't think you'd be missing out on anything great starting from St. Thomas. Your first seven night charter will be wonderful. However, there is no way you will see everything great you will hear about. Even with a longer charter you wouldn't hit everything so just be prepared to go again, to revisit your favorites and see a few new things. And then again, and again, and again.....