HankB
is Blowin Smoke!
That's a good choice IMO for your needs. If you occasionally want a smaller cook, you could also add a mini-WSM which would be good for a couple medium/small butts or birds.
IMO, better to be moderately oversized for your everyday needs than drastically undersized for your occasional needs.
Better to have and not need vs. need and not have!
You can always buy the charcoal ring for the 18.5" WSM to reduce the amount of charcoal needed for shorter cooks. It's rare to hear some say "I wish I had bought a smaller pit" but you hear "I wish i had a bigger pit" all of the time.