They’re not interchangeable. Either will work just fine for your sanitation needs, but they are not equals when it comes to sulfiting your wine. The Na form is reported to give your wine an off taste, even at the low levels used in winemaking. The K form should be your choice to use as an additive to your wine. I use strictly the KMS and figure why even keep the sodium around, just potential for a mistake to occur.
potasium metabisulphate is the best to use period, sodium metabisulphate can produce salt as a by product, which is something you dont want, I am in the uk, but, I believe / suspect us legislation favours potasium over sodium, at least as far as I am aware.