I'm late to this party, but I kinda wish I had the 22 as well. My budget was stretched pretty thin when I bought the 18......so I simply didn't have the extra $100 for the 22.
However, I like to put turkeys in an aluminum pan before smoking them. I usually have to finagle the lid onto the 18 in order to get it to fit around the pan. The 22 would make this easier. Racks of ribs would be easier on the 22 as well.
But.......life is still pretty good with the 18. I can't complain.