Is there any practical difference between potassium metabisulphite and sodium metabisulfate? Is there any reason you can't or shouldn't substitute one for the other?
They will both accomplish the same purpose as an antibacterial agent to kill yeast and bacteria and prevent oxidation along with other identical benefits, however I would recommend potassium bisulfite over sodium metabisulfate as it will not add sodium to the mead. (sodium, of course, is salt)
I use my sodium metabisufate as an equipment and bottle sterilizer only.