The difference is that someone is not legally forced to accept the inferior currency. All your attempts to horde the better currency can fail when the person you want to buy from either refuses the inferior currency, or accepts it, but at a price that accounts for the difference in relative value so there is no difference.

Edit, this used to say, but that is wrong: The difference is that the which is best changes over time, so you change which one you are hording based on conditions.