Some good things already said here.
An 18WSM is plenty for most people. I have two 18WSM's I purchased used and if I ever needed more room than one could provide, I would use the other. They are easy to find used online at a great price (I am currently trying to point my buddy to an 18 I found a few miles away from him for $40).
I have a Weber hanging rack for one of my 18's so its basically set up like a PBC. 18's are a great size for my current set up. The only way I could justify a 22WSM is if (and potentially when) I start doing BBQ competitions. You can do everything on an 18 that can be done on a 22...yes even full racks of ribs. You just can't lay them flat without cutting. That's all.
@swamprb makes good points as well. With a 22WSM you'll never have to worry about room. But I can't tell you how many people buy one because of all of the "cookouts" or "people" they are going to have, cook on them once or twice, and then sell them because they are "too big." If cost isn't much of a factor, buy a 22. If you're looking to get your feet wet, find a used 18 and play around with it. Its very easy to use and dial in.
Hell, my dad got an 18WSM for his birthday, and he has only used it twice in the past 7 months. I use my 18WSM every week/every other week.