Have got to agree with SCOTTYR on this one. For the money, the stock system is the best. Aftermarket systems are lighter, but LOUDER. Depends on your riding ability, public or private land, and the justification for less weight. Professionals can use them, but for the common man, stock is the way to go..
This question has been going around and around for the past year and still does not have a solid answer.
Read prior posts on set-up techniques and practices and you will be a lot better off. Don't forget to replace the stock crimped hose clamps with automotive hose clamps that you can screw on tight so they don't fall off whille your out riding.