You will notice a big difference with the 6 speed. I don't know when they started putting the engine/transmission brake on the gas engines but that was a big improvement as well. The integrated brake controller and the camper style mirrors are two options I'll always have from here forward. I didn't think they would be a big deal but I really love them. My '04 was a left over and my current '11 was as well.
Be sure to ask if GM is kicking in any clearance cash for the 13's. They often do give the dealer some assistance on leftovers.