I may be wrong but these are my thoughts.
Kent Marine Nitrate sponge is Zeolith base(stone base), similar to Seachem Denitrate.
Pura is a synethic resin, similar to Seachem Purigen.
I have tried both Seachem Purigena n denitrate. Both does not work as claim. I give up on them and stick to protien base denitration(Aquamedic).
My Bro MT have done another review of S base denitrator, it works like a charm but the cost is like
So I do agree w Bro MT when he say good things dont come cheap. Sori, .
Having said that, I know that there are some bros who did a bulk purchase in another fish forum, maybe u can get them to share their reviews.