I think the 18" may be what you want. I went with the 22" because I wanted quiet, and that's what I definitely got. No louder at idle, and quiet at cruise, but just a deeper tone on accelleration. Hard to tell from inside the truck that anything was done at all. Outside, my friend noticed the difference, but again, just a little bit "meatier sounding." If you didn't know it was a different muffler, you might not notice.
Often I wish I'd gone with the 18" because I'd like a little more "growl," but when I'm towing the trailer to go camping with the wife, I remember why I like the 22, as well. I think she's happier without the added noise (and, therefore, I am happier, too!).
Andy