As far as lowering that bodyfat, diet is the key factor, followed by exercise. Formerly sedentary people should start at the lower end. But for those people used to exercise, working at the higher ends (not anaerobic, except occasionally) will burn more calories overall during the exercise itself, and higher end intensities provide and "afterburn" where the body uses calories to replenish glycogen stores, repair muscle tears etc.
Fixating on using fat as a substrate (ie. working at the lower heart rates) is good for new exercisers or those coming back from an injury or illness, but I often tell my clients, that when you're lying on the couch eating Doritos, your body is using fat as a substrate.
