Well, Bill, to state the obvious- potassium meta (K-meta) has potassium and sodium meta has sodium.
That's the only difference. People who are sensitive to salt and sodium in general should buy the k-meta.
It's used for two basic things in winemaking- for sulfiting the wine to preserve it and to inhibit oxidation- particularly for wines that will be aged more than a year or two. This is done at about 50 ppm (I don't have an so2 tester, so I just do the "every other racking" and at bottling additions) which is undetectable at that concentration. When you bottle, you can add it (along with k-sorbate) to help ensure that refermentation doesn't start if you are sweetening the wine. This is usually referred to as stabilization.
The other thing it's used for is sanitation. I usually use one-step for my equipment, but some just wash the equipment with a wine making cleanser and then sulfite for the sanitizing step.