A kettle is a great grill and a meh smoker.
A WSM is a great smoker and a meh grill.
If you really want to stick with just one unit, you can improve your kettle's smoking performance and capacity with a smoke stacker kit. But the WSM will always be a better smoker. Especially on long cooks. You ever done an overnight cook with your kettle?
Few people really need the extra capacity of the 22 WSM unless catering a wedding. An 18 can do four pork butts or two briskets. And even more if you rig up a third grate. How often would you need more than that (and which couldn't be accomodated in a pinch by using your kettle as a back up)?
So imo the reasons to get the 22 over the 18 are:
1. Ribs. If you want to cook full slabs of ribs laid out flat, get the 22. Full slabs on the 18 require some kind of work around -- rolling, rib rack, cutting in half, hanging. I like to hang ribs, so my 18 works great. My typical rib cook is six half racks hanging. I can fit more than that. And I can hang full slabs in the 18 (although the bottom end does get kinda close to the fire). If you hang ribs in the 22, you can probably fit 97 full slabs -- or more!
2. Grilling. Because of the 22 inch form factor, the WSM 22 is better than the 18 when used as a grill.
If you don't care about 1 and 2, get the 18.