i would say:
if you don't have an 18" WSM, and you don't plan to travel with the stacker, get the 22
The 22 is perfect for ribs, butts, and briskets
the 18 won't hold ribs without cutting or rolling, and won't hold a full sized brisky
But if you mostly smoke chicken, tritips, and butts, don't mind rolling or cutting ribs, or mostly smoke for yourself, you can get by with the 18
That said, during my recent testings of the stacker (despite owning one for several years already), i've discovered that it works great for GRILLING indirect as well as smoking.
High heat indirect with the stacker is amazing. Fishes, chicken, root veggies, etc. The stacker is freaking perfect for stuff like that.
The thing was designed for smoking, but it goes way beyond that.