Hi Jon,
The availability of 22.5 is certainly better around the world and I think for the most part have better all around performance for on-road applications. I have a FUSO based camper which I had 19.5" Toyo M608Z tires on alloy rims. I switched to Toyo 16" Open Country tires and found they aired down much better and provided a softer ride ( leaf springs and cabovers can be rough) the draw back was they were not nearly as robust as the truck tires. In someways you really can't compare the two as the M608z were medium truck tires and the Open Countrys were really for a light truck.
Still, I know the XZLs will air down better, may provide a better ride, and will definitely be better in the mud. I will definitely make the switch when I find the right rims. i would prefer hutchinson but understand they are currently not available. I am going to be looking into that.
Eric