The 18" is slightly more efficient and lighter so it's easier to move around.
However, I would get the 22" if I only had to have 1. The extra space is wonderful. If you want a fuel efficient small smoker, build the mini tamale pot.
One day I am going to do 4 butts, 2 on each, starting with the same amount of charcoal and give an accurate comparison on fuel usage. I say the 22 uses more, but I am usually cooking A LOT of meat on the 22 where as usually only 2 butts or a few racks on the 18. Also, my 18 is very seasoned and sealed, not so much with the 22".