Without being completely positive about it, I would make the assumption that in the case of brackish water fish it would be better to use a marine salt mix as it would also provide the trace elements and minerals in the proper amounts that these fish would encounter in their natural environment.
I would further assume that the same would also hold true for non-brackish fish, yet probably not as vital as it is for brackish water critters.
If you are going to use salt in your freshwater tank, I can't think of any solid reasons to not use marine salt mix.
I would avoid mixes intended for reef tanks though, as they usually have higher levels of certain elements that could cumulatively impact water quality as those elements would not be utilized by living critters such as coral and would accumulate over time and increase the TDS of the water.