Galvanized steel is steel that has been surface treated with zinc so that it has a level of corrosion resistance. You see this kind of steel all the time in things like chain link fences, where they are out in the elements all day. All the galvanized chains are, in fact, hand coiled, cut and assembled from electric fence wire (yes, I do every ring by hand). It’s an affordable metal, but still strong enough to hold up to most abuse, it’s just not intended to be load bearing. Now, if galvanized steel gets wet (rain, sweet, etc) rather than rust it will pick up a surface tarnish and won’t be as shiny. Some people like the look. Or, to an extent, constant handling will polish the surface and give the chain a slightly darker gunmetal look. Galvanized steel is a metal that, in a way, can be seasoned or neglected.
Stainless steel has a higher chrome content, which makes it much, much harder. As its name indicates, it will never stain, rust, or tarnish, and maintain it’s glint (I’d say forever but I’m not sure I’ll live long enough to find out). These chains are considerably more expensive than normal steel ones. For one thing, they take near twice as long to make. The hardness of the metal resists bending and requires more effort. Two; stainless wire is not as accessible as galvanized steel wire, so I have no choice but to order the rings pre-made. It’s small, but it’s still an added expense. But in the end, you get a wallet chain that–unless something drastic happens like a train runs over it–will outlive all others.