A Mars bar is not food. Don't confuse that with a banana which is food. Any discussion going forward has to start with this. At any rate, the sugar in a Mars bar is a combination glucose and fructose, probably in similar ratios to a banana.
Fructose is processed directly by the liver without causing an insulin spike. It is very good at replenishing glycogen, which is important if you need to be able to efficiently use your muscles. Glucose will spike your insulin which, in the presence of protein, will help protect your muscle. The before or after depends on your training goals. Before gives you more endurance during the actual exercise, after, helps your recovery after the exercise. Like in most things, the right amount is just right and too much is too much. It's the too much that leads to fat gain. How much is too much varies for each person.
I'm going to have to disagree with you on this, fructose is extremely poor at replenishing muscle glycogen and very good at replenishing liver glycogen.