Last season we had lots of posts from people that added Clorox brand salt to their pools and the results were lots of yellow/brown stains. Salt can often be found to have sodium or potassium ferrocyanide in it as an anti-caking agent. Chlorine breaks down the ferrocyanide and releases iron.
Water softener salt is better because, by its intended use in resin bed softeners, it has to have zero iron content in it or else it will irreversibly damage the ion exchange resin. So, from a purity standpoint, softener salt is better.