If you want to add muscle and lose FAT during the same workout you should do the weight training first. Why? First of all you'll have more energy, which usually results in a more productive weight training workout. Second, there is a time span of about 60 minutes, after starting the workout, where Growth Hormone levels are slightly higher than normal. You want to take advantage of that by making the workout not last more than 60 minutes. Weight training first may hamper the aerobic exercise because your tired, but you raise your chances of building muscle, which will burn more FAT in the long run. A better way would be to do them on different days and allowing yourself adequate rest between weight training sessions.