If you have the ability to, I'd say go with the 22. You can always do smaller cooks with your 22 but you can't do bigger cooks with the 18.
WSM 22 ~ WSM 18 ~ Maverick ET-732 ~ Flaming Fast Thermapen ~ Weber SS Performer ~ Weber 22 OTS ~ Weber 22 OTS ~ Weber 22 OTPlus ~ Weber 22 OTS ~ Weber Q300
When all else fails just ask yourself, WWGALD???