Salted vs Unsalted Butter

Here is some information I found in my Cooks Illustrated Magazine:

The exact amount of salt varies by brand, ranging from 1.25 to 1.75 percent by
weight. So using the average of 1.5%, there are 1.7 grams salt per one-fourth pound stick of butter, or a volume of 1/3 teaspoon.
In a taste test between salted and unsalted butter, it was found that differences were subtle in food with strong flavor or many other ingredients (like yellow cake or chocolate chip cookies). However, food that required a butter flavor, such as pie crust and buttercream, tasted "harsh" and "overwhelming" when salted butter was used. The 2 problems with salted butter are it is hard to control the amount of salt and it can be stored longer, so it may be less fresh (although the salt may mask off flavors that the heat of cooking exposes). Recommendations: it is possible to substitute in some recipes, but judge the dish carefully and don't use salted butter in those where butter is the primary flavoring.
A personal note: I rarely use unsalted butter because it is nearly 2x the price of salted and it does not keep long. Some unsalted butters that were tested had gone bad before the expiration date!
I hope this info helps you out!