Short answer -
WSM (and 18" unless you need the extra capacity of the 22" - if you don't, then save the money).
If you could only have "one" to do it all, then the ceramic cooker makes sense. But since you already have a
Weber kettle and are comfortable with that for higher temp grilling, then unless you are going to be limited to only having one grill/smoker due to space, I would go with a
WSM in addition to
the kettle.
I have a 18" and they did not have the 22" model back when I got mine. There are pluses and minuses to either, the 22" will require a bigger load of fuel and go through it faster (more volume to keep at temp), but it holds more food and you don't have to cut or coil your ribs like you do in the 18" model. That being said, I am not getting rid of my 18" either. If price is an issue there is nothing wrong with a 18"
WSM unless you really need the extra smoking capacity of the 22". I regularly smoke 4 large pork shoulders at a time in my 18"
WSM. I did 6 once so it can be done (it was a tight fit and I had to stand them on end on both levels).
Also for either a WSM or a ceramic cooker used as a smoker, I would highly recommend a power draft such as the BBQ Guru models. I have a DigiQ-2 and it makes a long smoke about as close as "set it and forget it" as can be done in a smoker. It's basically a computer controlled air supply (you block off all the air intakes but the one from the power draft fan). It either stokes the fire or chokes it back to keep the pit temp at a set point. I've held 225* for 12 hours without touching a thing in my WSM using the Guru. Learn how the pit works "as is" first, but at some point go with a power draft (there are several brands out there). You will not regret it.