All that PSB and PNSB really have in common is that they're both photosynthetic. In others words, they're no more closely related than seaweeds are to pine trees. In fact, in this comparison, these distantly related groups of bacteria differ metabolically from one another even more than the seaweeds/pine trees are.
So it really depends on what you intend to use them for (the specific traits you hope to substitute with PNSB). There is some crossover. Both are good as food. Both have probiotic qualities (though they are not necessarily entirely the same). Both inhibit pathogens, including Vibrio. Most if not all species in both groups are capable of carrying out diazotrophy. Both are pretty good at degrading organic wastes.
I'm fairly familiar with both groups...more so with PNSB, but if you know the PSB species in the product, I'd be happy to help you determine its effectiveness as a substitute (assuming the contents of the product are accurately labeled).