The obvious answer would be salt, right? Not so fast. The scone recipe below and some recent baking with a friend brought up the issue of salted versus unsalted butter, and I found myself explaining how salt is not the only difference between the two.
Salt is a preservative, and salted butter can last two to three months longer in the refrigerator than unsalted butter. So this actually means that salted butter is often much less fresh than unsalted, and sometimes has been made from cream that is less fresh as well.
Overall, it's best to buy and use only unsalted butter for cooking and baking, especially since you can't even reliably determine how much salt is in any given stick. Ochef gives an estimate of as much as 3/4 teaspoon can be in a stick of salted butter, but this varies depending on brand and place of origin.
So you're better off with unsalted, except for your morning toast and muffin. To keep it fresh longer, you can always store it in the freezer.