This sounds comparison is pretty accurate. Since the Hellas are dual tone, they are louder and grab your attention more. They seem a lot louder through the cab too. They'll definitely draw more attention than the stock.
I got mine from Pep Boys, so it cost me $60, but I had them right away and didn't have to pay for shipping.
I'd recommend getting some of the tubing as well. Helps protect the word from dust and the elements. It only costs a few bones.