I've paid attention to Calvin Elston over the years- he may be one of, if not THE most talented behind the scenes guys in exhaust science and header fabrication.
In his opinion, the most important factors in exhaust design (for a given engine combination) are-
-pipe diameter, bends immediately after the exhaust port and collector design. <<Funny that; he had no access to available, modern science when he was kicking tail in Nascar...but today it's proven ole Calvin was on the money. He took a lot of grief back in the day from the "in the know" crowd, lol.
I'll add that I've made additional power (as Sean and Scooter mentioned) by using an X-pipe- location is important, so I usually put spray paint in the area where I'm thinking about building it. Where it immediately burns off is where it goes.
I'm going with the Dynomax with the built in X-pipe. I'm a fan of Magnaflow, too.