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 plays a minor role, whereas the sodium salt is highly accessible. Mercaptoethanesulfonate includes both a thiol, which is the primary reactive center, and a sulfonate group responsible for aqueous solubility.