The sodium and potassium ions don't add or detract from the function of the metabisulfite. Use the one which you don't mind having a trace of that particular salt in your beer.
I bought a pound of the k-meta powder on Amazon last year. By my rough estimates, I might run out in about 2060. Just don't ever inhale when you open the container, hooboy it's like inhaling about 40 crushed sulfur matches, yuck.