AJ, I have a 18 WSM, and picked up a hanging rack for Father's Day. It's a great accessory-- I really love it-- but it's not as useful as I had hoped. My thinking was that it would allow me to hang bigger pieces of meat than I could lay across the grates, but that was an optical illusion. The 18" dimension across the grate is longer than the distance between the top of the rack and the water pan. So while it does make it easier to hang racks of ribs, the racks need to be shortened a bit to fit. The ability to hang whole fish inside is unmatched, though. And hanging a whole chicken or a turkey breast inside yields a really beautiful and juicy product.
(For the record, I can fit 8 racks of ribs on my smoker if they are 'rolled' (curled into a 'C' with an overbite, and held together with 2 or 3 bamboo skewers). This is a pretty efficient method, but doesn't yield the prettiest product on the plate. So I'll prolly continue to use the hanging rack for ribs. I got the rack and the extra pack of hanging hooks, so I recently hung 6 racks, plus the remaining cutoff ends, and had room to spare. )
To your larger question, I definitely prefer the 18, since one of my favorite things to make is a single loaf of smoked meatloaf, and I would hate to fire up a huge 22 to do it. It also comfortably cooks 2 shoulders, which is a heck of a lot of meat to feed a crowd. But if I came across a 22 WSM and had the room to store it, I'd love to have it for briskets and bigger cooks.
...and as others have mentioned, don't overlook the smoking potential of your kettles. That's still my go-to for shorter smokes like fish, chicken wings, turkeys, and vegetables. If you're cooking for a big crowd, you can do a lot of smoking on one or more of your kettles.