ChadVKealey
TVWBB Pro
It sounds like the 18.5" WSM is the best choice if you're planning to focus on low-and-slow cooked foods. I'm going to throw another wrench in the works and suggest that if you find yourself wanting to do some occasional charcoal grilling, you pick yourself up an 18.5" Jumbo Joe ($60) or 22" Jumbo Joe Premium ($68, I think, at Walmart). Either one of them will let you cook up burgers, dogs, chicken, or other grillable goods for 3-4 people. They both have a smaller coal-to-grate distance than the standard kettles, so they're a bit better for direct grilling.
That being said, I think the combination of an 18.5" WSM and a 22" Performer is hard to beat. Between those two, used alone or in combination, there's very little you can't do.
That being said, I think the combination of an 18.5" WSM and a 22" Performer is hard to beat. Between those two, used alone or in combination, there's very little you can't do.