He wrote this in 2008:
I do not believe it is publicly known exactly what the Purigen polymer is. But many of these act in similar ways. These sorts of polymers will bind organic materials, and may bind a slightly different subset of the organics in reef aquarium water than will GAC, so I'd say that to some extent they can be complimentary and using both is a fine plan.
I use GAC for my system, but cannot demonstrate that it is better than Purigen in reef aquarium water.
I’ve tried just Purigen and it doesent keep the tank clean for long. Gets clogged quick. When you use GAC and it I feel like it also extends the life of both. They clean each other’s overlap. I find I have to feed often to just keep my nitrate and phos above 0. I now just grab a bag of chemi pure blue 75g. Throw it in the sump before the return, works like a charm. Then every few months run gfo to remove the aluminum the purigen puts in.