I have read a number of sources that recommend the use of potassium metabisulphite or potassium-based campden tablets in preference to their sodium-based equivalents (e.g. Jack Keller states that he doesn't recommend the use of sodium metabisulphite here), but I have been unable to find a clear explanation of why this is. And it seems that most American recipes that specifically call for one or the other suggest potassium. But here in the UK, it is by far easier to acquire sodium-based sulphites. As far as I can see, they are very similar and produce almost identical reactions in the wine, so is there any reason I should take the extra effort to acquire potassium-based sulphites?
Primarily its a subject of dietary preference to avoid additional sodium in the diet. However, its a moot point because generally a very small amount of sodium would be being added.
Also potassium has a somewhat higher flavor threshold than sodium. Meaning sodium begins to taste saltier sooner than potassium. (at least this is the case on my palate)
Lastly, the dissolution of the potassium salt is slightly faster than the sodium salt, leading to an almost trivial increase in potency of the potassium v. the sodium.
I am just pointing out the differences. I wouldn't shy away from either form based on what was available to me.