I would go with the 22. It is what I have been running for two years. It has been great for everything from as small as two racks of spares up to 30# of brisket and 20# of butts. It could have held more, but that is the most I have put on it at once. It can also hold large briskets (17#+) or several rib racks flat on each level.
I also made a smaller charcoal ring out of expanded metal to do the smaller cooks.