This thread could easily turn into another chemical purity debate. That's not entirely unjustified, since that's the only likely difference between the two, from a chemical perspective. My notes indicate that The Chemistry Store sells the pentahydrate form -- but that's from a few years ago. I've used their sodium thiosulfate with no obvious ill effects, but lately I've been using ammonium thiosulfate-based fixers instead of those based on sodium thiosulfate. Ammonium thiosulfate fixers are less expensive, on average, and they work faster.