Wow, sorry I missed this SP! Welcome to SMF and my apologies you didn't get an answer sooner.
The WSM is probably the best, most dependable out of the box smoker you can buy. Readily available supply of fuel and wood too.
As far as the 18.5" vs the 22.5", dealer's choice based upon your budget. It is just my wife and me and I chose the 22.5". It uses probably 30% more fuel than the 18.5" because there is a larger space to heat, but buying charcoal in bulk from the big box stores on Memorial Day, July 4h, and Labor Day makes that cost difference minimal. Both sizes are great choices and can handle the volume for smoking for just the two of you or large parties.
With the 22.5" I only use the lower grate on occasion and don't have to trim any of the cuts of meat I buy. All my roasting pans fit too. I can slap an 18- 23 lb turkey on the top grate and my favorite roasting pan filled with veggies on the bottom grate.
Whichever one you choose you'll be happy. The WSM is one great smoker.