The cation is unimportant, but the sodium salt is most available. Mercaptoethanesulfonate contains both a thiol, which is the main site of reactivity, and a sulfonate group, which confers solubility in aqueous media.
The cation isn't significant, but the sodium salt is most readily available. Mercaptoethanesulfonate contains both a thiol, acting as the main reactive site, and a sulfonate group that provides solubility in water.