From the extensive research and reviews I looked at a while back, SDS could not be beat.
They have solid prices and seem to know absolutely EVERYTHING about sound deadening in automobiles. Their website will tell you everything from what product you'll need to buy, quantity, and where to apply it.
Their website isn't as simple as it used to be, but if you can read all of the information and understand why they're the best, you'll do it right the first time and save a lot money. I did my whole trunk and deck in my last Mustang for only $40.