I often have to dome my briskets (I love cooking 16 pounders because the flat thickness is consistent) on my 18 but that doesn't seem to make a big difference.
That being said, charcoal is cheap and I wouldn't be complaining one bit if I had the larger one. I just decided to go with a UDS instead.
Old user name was JP7794
Jim - KCBS CBJ
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18.5" WSM (1983 model) | 18.5" WSM (2010 model) | Packer Green Thermapen
| Auber PID Temp Controller | QMaster Senior Temp Controller | Mini WSM | Green Bay Packer UDS