So, which is best for losing body weight: cardio or weights?
The answer is not so clear-cut. But [normally] when people talk about wanting to 'lose weight', they don't want to lose muscle; instead, they want to flick the flab.
Focusing on weight training will boost muscle and blast body fat simultaneously, but if your muscle and body fat change by the same amount, your body weight (according to the scale) may stay the same, even though you get fitter.
Strictly focusing on cardio, on the other hand, chances are you'll lose body weight (mass), but this tends to be a combination of fat and muscle.
So it's pretty fair to say, doing a combination of cardio and weights, ideally in the one session, may be best for improving your body composition. Typically this can be achieved via interval circuits — specifically high-intensity interval training (HIIT) workouts that include cardio and strength moves — as the most effective and efficient way to maximise fat loss and build lean muscle.
In particular, exercises that focus on "compound" movements, such as moving between squats to burpees to lunges, over isolated movements like crunches or bicep curls, means you'll employ more muscles at once, which increases exertion and burns more calories in half the time.
However, this type of training is not suited to everyone. You'll need a fairly good base level of fitness to undergo such explosive type movements. What's more, vigorous-type training is not to be carried out on consecutives days, as the body needs sufficient time to recover from such intense-type activity. This doesn't mean you stay on the couch in between workouts. Lighter activities like brisk-walking (>30 minutes) is recommended for weight loss on most days of the week, and also serves as a good base to build up fitness.
The Takeaway
For fat loss (and health), the ideal exercise program includes a combination of cardio and weights. It's also important to remember what and how much you eat has a far greater impact on how much weight you lose.