Campden tablets may be made either from potassium or sodium metabisulphite. Generally, they come in a 0.44g tablet weight. However, I've personally weighed a batch of 25 that had three different weight groups.
Regardless of the metallic ion, the metabisulfite part produces the SO2 that is needed at various points in the brewing process.
Many people today are concerned about being heart-healthy - low sodium intake. That is the main reason why one might avoid the use of a sodium based compound. Another reason is that sodium based compounds can impart a salty taste to your "beverage". The amount needed will vary depending upon one's taste sensitivity.