I'll start of saying that I think the WSM 22.5" is the most awsome bullet smoker ever manufactured. They are just the bomb when it comes to volume or even competition cooking.
That said I do think the WSM 18.5 is a better fit when the extra space is not a neccasity. I do not own a WSM 22.5...YET, lol. But I do own a Pro Q Excell 20 and I can tell that by comparrison between these 2 that while both are quality smokers, for my needs generaly speaking the WSM suites me better due to the fact that it takes longer for the larger Pro Q to come up to heat and the vent adjustments are quicker to respond on the WSM because of it's smaller size. Also I notice that the WSM is more efficient with charcoal, it uses less because it takes it less time to adjust and come up to heat. Now maintaining heat once they are both there, I'm not sure if ther is a difference.
Another example is I have a Weber SJP (jumbo Joe) and I have fitted a Pro Q WSM stacker to it and I can convert it from a grill to a sweet little smoker in a matter of minutes, and it comes up to heat and is faster to react to vent changes than my 18.5 WSM totaly due to its smaller size.
If I had to pick just one or the other based on everything, price,efficiancy, and application I would go with the 18.5
Joe