I have two 18.5" WSMs. I only have tow of the same because I found a good deal on a used on on craigslist and didnt want to spend the money on a 22.5. If you are going to only have one, get the 22.5. You will be sorry (as i was) if you get the 18.5 and then want to cook for more people. For instance with rib racks, I can only get about 8 racks on one 18.5". Hope this helps.
Weber Genesis Silver B, Twin 18.5" WSMs, Weber Performer, Green Weber Smokey Joe Gold