I'm not an engineer nor a technician, so please keep that in mind... but this write up on filter system design for breathing air http://www.scubaengineer.com/high_pressure_compressor_filtrat.htm will give you a good idea of what it takes to get clean and dry air into our tanks which are designed to hold breathable quality air.Our air doesn't have to be exactly breathable (odors, carbon dioxide ok), but it must be dry. The problem is that neither the Diablo nor Joe's have any kind of technical data so judging their effectiveness is difficult.
so I wonder what the pros and cons of intake vs output drying? I assume that the desicant would last longer on the output so long as the bleed valve was opened occasionally. both would probably be best I assume