Brad, what are the differences you speak of. I just bought a 22.5" and was told that they were identical in every way, except for size.
Jeff didnt see this post, I have a 22.5 wsm myself and a friend of mine has the 18 and the biggest difference is fuel consumption. Mine spends charcoal like congress spends on a budget. If I could have a do-over I think I might've swinged to the 18. It's nice to have the extra room if need be but I ran a Super Bowl party with 18 people and that'll probably be the biggest crowd I'll feed. Normal use is for family 5-7 people? So what I plan to normally cook isn't enough to rationalize the consumption in fuel.