18.5" for us, there's room for three 8-10lb pork butts per grate and a 11lb brisket per grate. A lot of people like being able to lay 3 racks of ribs on the 22.5" , but it's not a big deal to me to use a rib rack, curl the ribs, or cut the ribs. I cut them up for eating anyway. You'll also read about using the 18.5" charcoal ring in the 22.5", and the popularity of the mini smokers and the new Weber 14.5". What's that really telling us? I'm just saying..... But it all comes down to getting what you'll be happy with, and only you can figure that out.