Margarine or Butter?
If you have to ask, then you should know that butter is best. I shiver to imagine what margarine based toffee would be like. Shortening has a place for some pie crusts, but when it comes to cookies, I believe you need real butter. Great food comes from great ingredients. If you are concerned about health and trans fats, then don't eat a lot of cookies or toffee. You can always give them away.
Always buy good quality butter and don't store it for weeks. Freshness is always paramount. Just about any good quality butter will work, but I don't recommend European style butter for toffee. It is supposed to be good for baking (I haven't tried it), but it was a complete disaster for toffee. Fortunately, the problems were apparent well before the work of heavy stirring or dipping. After adding the chopped almonds to the hot mixture, the mixture separated into liquid fat and almond paste. No amount of stirring would restore the toffee to proper consistency.
Salted or Unsalted?
Unless specified in a recipe, I always use salted butter. I don't understand the fuss everyone makes over controlling the amount of salt. I guess I can understand that if you don't want any salt in your dish—or maybe just a little salt—but I have faith that butter producers put a consistent amount of salt in their product. Who tastes their cookie dough to announce, "just a pinch more salt"?
