The 07 and later touring mufflers were redesigned and have a better tone quality which is deeper but not necessarily a lot louder. They are definitely just a bit louder than previous year stock slip ons but do not approach the sound level of aftermarket pipes by any means. I happen to like the sound over older mufflers.
If you check ebay, there's a lot of the 07 and later mufflers available at a good price from touring bikes.
The basic difference between the touring mufflers is that the FLHX (street glide) set is slash cut in the opposite direction than the Road King and there are other slip ons that have the straight cut ends on other models if you look around. It depends on your preference.