Programs for weight loss ideally include dietary restriction and an increase in physical activity. Physical activity can include formal exercise or an increase in incidental movement, preferably both.
Formal exercise programs include sports, regular running or jogging, or training programs and sessions in the gym or health club, which range from weight training to aerobic sessions, or a combination of both under a personal trainer or gym instructor.
Ultimately, the contribution to weight loss (preferably fat loss), of any discipline is in the energy expenditure. Even so, there may be benefits and deficits of particular exercise regimens.
The fundamental and irrefutable premise of physical activity for weight loss is that you must create an energy deficit, which means the energy you consume in the form of food and drink must be less than you expend in resting metabolism plus incidental movement and deliberate physical activity. This is not in dispute, even though you may read otherwise in popular web sites and magazine. 'Energy in, energy out' is a fundamental of physics and it applies to weight loss as well.
That's not to say that various conditions don't influence energy factors. For example, some foods like protein, have slightly higher digestion and metabolism requirements, but it's only a small advantage compared to carbohydrates. High-intensity exercise tends to leave your metabolism higher after you exercise (the afterburn) but you have to balance that against the quantity of energy expended during the exercise session.
Let's look at how aerobic (cardio) and weight training stack up against each other.
Aerobic, Cardio Training
Cardio training like running, jogging, cycling, rowing, swimming, group aerobics or even fast walking is characterized by regular, repeated movements at a level of intensity that allows a sustainable use of oxygen. ('Anaerobic' means exceeding a sustainable oxygen requirement.)
Cardio training promotes aerobic fitness and heart health because of the dynamic changes the occur to the heart muscle and the blood vessels.
Cardio training tends to be non-stop and usually for a time greater than about 20 minutes, and perhaps up to an hour or more. The non-stop nature of cardio tends to accumulate extra energy expenditure in comparison to interrupted training such as weight training or intervals, even when you account for the energy expended 'after' your session has concluded.
Weight Training, Resistance Training
A full-body weight training program can include up to 10 exercises with 3 sets of 12 repetitions of each exercise, which may take an hour or more to complete. It sounds like a vigorous workout, and if you lift heavy enough, it can be. Even so, because you rest for 30 seconds to 2 minutes after each set -- let's say 1 minute -- you are effectively not moving for around 30 minutes of the session. This makes a substantial difference to energy expenditure and fat burning, even if your workout is vigorous with heavy loads.
However, high-intensity exercise can create post-exercise energy expenditure (the afterburn) as your metabolism stays higher for several hours after your workout. While this is useful, it is unlikely to match the energy expenditure of an hour of exercise in which you move at a moderate pace without stopping.
An advantage of weight training is that during a weight loss program, it can help you maintain muscle mass, which is important in arresting a decline in metabolism as you lose fat.
An hour of running or jogging at moderate pace can burn around 800-1000 kcalories. An hour of vigorous weight training in the gym might burn around 500-600 kcalories. Continuous movement, as a general rule, always burns more calories than interrupted physical activity. Circuit training, which usually involves continuous movement between weights workstations, can increase energy expenditures substantially.
The best strategy is to combine an aerobic workout with a dedicated weight training session. These need not be on the same day or the same session. In fact, it may be preferable if they're scheduled for separate days or at least morning and afternoon session of the same day. This way, you approach each session with replete energy systems which enables you to give you maximum effort.