Contrary to what many people believe, weight training is much more effective than aerobics for fat burning.

 
When you increase your muscle mass through resistance training, you increase your Metabolic Rate, or the rate at which you burn calories . For every 3 pounds of muscle you gain, you’ll burn an extra 120 calories a day. Over the course of a year, that’s about 10 pounds of fat. This means you’re burning extra body fat even when you’re sitting down, because muscle takes more energy to sustain.

When you do aerobic training, you may burn more calories during the workout. But this is only marginal (about 20% more!).
Also aerobic training doesn’t have the advantage of shaping the muscles like weight training does. In fact, long duration aerobic training may cause muscle loss as well as fat loss (just look at the shape of Olympic Marathon runners!)

….and now there is scientific research to back this up;

1.Schuenke MD, Mikat RP, McBride JM.
Effect of an acute period of resistance exercise on excess post-exercise oxygen consumption: implications for body mass management.
Eur J Appl Physiol. 2002 Mar;86(5):411-7. Epub 2002 Jan 29.
In this study they used circuit training consisting of 12 sets in 31 minutes. EPOC (excess post-exercise oxygen consumption) was elevated significantly for 38 hours post-workout.
Put another way, if you train 4 times per week your metabolism is pretty much elevated all the time!

 

2. Kramer, Volek et al.
Influence of exercise training on physiological and performance changes with weight loss in men. Med. Sci. Sports Exerc., Vol. 31, No. 9, pp. 1320-1329, 1999.
Overweight subjects were assigned to three groups: diet-only, diet plus aerobics, diet plus aerobics plus weights. The diet group lost 14.6 pounds of fat in 12 weeks. The aerobic group lost only one more pound (15.6 pounds) than the diet group (training was three times a week starting at 30 minutes and progressing to 50 minutes over the 12 weeks).
The weight-training group lost 21.1 pounds of fat (44% and 35% more than diet and aerobic only groups respectively). Basically, the addition of aerobic training didn’t result in any real world significant fat loss over dieting alone.
Thirty-six sessions of up to 50 minutes is a lot of work for one additional pound of fat loss. However, the addition of resistance training greatly accelerated fat loss results.

 
3. Bryner RW, Ullrich IH, Sauers J, Donley D, Hornsby G, Kolar M, Yeater R.
Effects of resistance vs. aerobic training combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate.
J Am Coll Nutr. 1999 Apr;18(2):115-21.

 
The aerobic group performed four hours of aerobics per week. The resistance training group performed 2-4 sets of 8-15 reps, 10 exercises, three times per week.

 
V02 max (oxygen consumption / aerobic fitness) increased equally in both groups. Both groups lost weight. The resistance training group lost significantly more fat and didn’t lose any LBM (Lean Body Mass), even at only 800 calories per day.

 
When on any weight loss programme, it is ideal to burn body fat and hold onto or increase our muscle mass. This is also true for ladies. It is important to note that in this case we are not talking about building large muscles but rather toning the muscles to shape the physique.

 
The resistance training group actually increased metabolism compared to the aerobic group, which decreased metabolism. This would explain why people who lose weight by aerobic training alone might not necessarily get into better shape. And they often find that their energy is not great also.
Short duration intense weight training gives better results all round. It boosts your metabolic rate, turning your body into a lean mean, fat burning machine. This also boosts your energy levels, boosts mental alertness and drive and believe it or not, because your heart rate is elevated for the 20 minutes, improves aerobic fitness too.

Unlike low intensity cardio, weight training has a significant boost on post-workout metabolism.

A study published in Eur J Appl Physiol. 2002 Mar;86(5):411-7. Epub 2002 Jan 29. showed that weight training caused a significant increase in metabolism for 38 hours post-workout.

 
Another study Med. Sci. Sports Exerc., Vol. 31, No. 9, pp. 1320-1329, 1999 was done comparing diet, diet and cardio, and diet plus cardio plus resistance.

The one who combined weight training with the other two factors lost the most body-fat. In fact, the cardio group only lost one more pound of fat than the diet only group.

 
In another study J Am Coll Nutr. 1999 Apr; 18(2):115-21, directly compares resistance only to cardio only.

The cardio group did 4 hours of low intensity cardio a week. The resistance group did 2-4 sets of 8-15 repetitions of weight training exercises for 10 exercises, 3 times a week. Both groups were on an 800 calorie deficit. Both lost fat, but the weight training group lost far more fat and unlike the cardio group, they maintained all their lean muscle.

As we can see from these studies, weight training and diet are the main factors behind fat loss, not cardio!!!