Dairy companies use salt to preserve butter. So salted butter may sit on the shelf longer than unsalted, which explains why unsalted butter has a fresher taste. You also never know exactly how much salt is added. As you can probably tell by these two points, I'm a big fan of unsalted butter for baking. On my toast, I'll take salted butter any day!
I hope this helps. Much luck to you!
That's exactly right, mimibu. That's why I always prefer unsalted. You know exactly what you're getting.