I've not used Advantage - but I can recommend Island Car Rental. http://www.islandcarrentals.com/Home.aspx
If you have a World Master Card, one of the very few cards that covers the CDW, it would be about $US200/week for August. They have fairly new cars in very good shape. The thing I liked about them was they have 24-hr road-side breakdown coverage anywhere on the island. Never needed it, but the peace of mind was nice.