Can someone provide a synopsis on the direct differences between resilient seated valves and metal seated valves (or provide a link to a good site that describes the differences between the two)?  What is the primary initiative to choosing one type or another for water (or wastewater) treatment plants and distribution systems?

Resilient seat valves are latest and greatest item for waterworks valves, been out for about 20-30 years, which is still considered newfangled to some operators
smooth waterway, dutile iron, (lightweight and strong) easy to replace the disk (as if anyone needs to).  If there is debris in the pinch area, the rubber (resilient seat) will deform to attempt a seal, otherwise flushing can clear the debris.  Stem sealing is by o-ring so near zero maintenance on the packing gland.

Metal seated valves have been on the block for the past 100 years, they still work, (if you know how to operate them)
Issues, there is a groove in the waterway that get cloggled with debris.  Regular exercise will keep the waterway clear, proper operation when the valve(globle valve) is needed will get a decent seal.  The trick is to use the water to wash away the debris.  They were built stout, servicing them is possible, the seats can be machined and disks, gears, threads and linkage replaced.  The stem seal is typically packing which needs to be kept wet to maintain the seal, (another proper operation requirement).  There are tools that can replace the packing though the valve box without having to dig up the valve.

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