There are many ways to get alot more meat on an 18.5. Rib racks, etc. If fuel is not a consideration, I say go with the 22. I can do up to a 16# full packer brisket on my 18.5 or 6 racks of ribs using rib racks (3 per shelf by bending them in a "U" shape) or 4 butts (2 per shelf). With about 12 lbs of KBB, I can run about 14 hours at 250*F +.
Turn a SS bowl upside down for a a large brisket:
Ribs on a rack:
You'll be happy either way. Good Luck!