The only difference is the salt.
Unsalted butter stales faster. And there is hearsay that manufacturers salt
old butter to salvage it, so unsalted butter is supposed to be fresher.
Different brands of butter carry different levels of salt, so unsalted
butter will guarantee that you can control the level of salt in your food.
Mostly people use the unsalted butter to make cake buttercreams, but I do not
find any significant difference, since I mostly use french butters which are
lightly salted.
Later,
Jazzbel