Answer: That would depend upon your goal. If fat loss is the primary goal then it’s a simple formula: the more work you perform, the more calories you will burn – the work being a combination of time and intensity.
Example: if you weight train for 30 minutes, rest only long enough to quickly get to the next exercise and go to failure each time you perform a set. Following this format you will have burned the greatest amount of calories possible from a 30 minute resistance training session, including the increase in your post-workout calorie burn (exercise-induced post-oxygen consumption [EPOC]).
Therefore, in this scenario, it doesn't matter how many reps you perform in a set or how much weight you use because you are constantly moving and maximizing your calorie burn per unit of time.
If your goal is to build muscle, your training requires continuous unaccustomed workouts and a weight and repetition scheme that provides sufficient time under tension. This is best accomplished with moderate reps of 6-12 and weight that allows that to occur (typically 75-85% of maximum). In addition, maximum muscle growth ultimately requires a higher volume of work than general fitness or moderate size increases. For a beginner or one interested in general fitness, three sets of per body part done three times a week is fine; however, as one continues to push muscle growth, total volume per muscle may be 6-12 sets done 1-2 times a week. Also, calories must be sufficient to allow muscle size increases and will need to increase as lean body mass (LBM) increases.