aloh makes a good point here. Talking with Kerry Bryant (president of Area P) yesterday he said that the difference in the 18" and 15" was +3 dbA on the 15". I didn't ask, but I'm sure that would be at idle and would probably break the 100 dbA at 5000RPM.I think you should be more concerned on how it sounds instead of how it looks. If you want it to be louder, then get the shorter one. If loud isnt for you, then go for the 18".
Thats just how I would make my decision... When youre riding the bike, you dont see the exhaust, you hear it.