Strong and weak nucleophicility, like ionic / covalent bonding, is not a yes / no concept. Especially when you're trying to get ideas of synthetic approaches, a mixture of compounds will often be formed. Having said that, because oxygen is generally more electronegative than sulfur, it is unlikely to want to share electrons (which is a basic idea behind nucleophilic attack). So ethaneselenol is likely to be an even stronger nucleophile, for instance.