One key observation that I have noticed, YMMV.
I have seen several instances where people bought an 18.5 and then later went out and bought a 22.5.
But fewer instances where people bought a 22.5 and then went out and bought an 18.5.
Put another way, show me a person who has an 18.5 and a 22.5, and more often than not, I can probably show you a person who bought their 18.5 first and decided later that they wanted the big dog after using the 18.5 for awhile.
I have an 18.5 and a 22.5. My 18.5 was purchased first.
The 18.5 gets little use now, and if I were doing it all over, I would have just bought the 22.5 and been done with it. Measure twice, cut once. For the extra 100 bucks, IMO its worth the difference.
It uses not much more fuel than the 18.5, and the extra space makes up for it's price difference.
I actually had access to two 18.5s, my own and another before I finally broke down and spent the 4 bones on the 22.5.
Figuring "I don't need the 22.5 when I have two 18.5s at my disposal." That was flawed logic.
No more getting creative rolling and pinning ribs, and no more stacking grates and working about hitting the top lid when it closed, being able to get a full brisket on it, all made it worth the difference for me.