I get asked all the time, "If I want to lose fat, should I be doing more cardio or weight lifting?". Well, that's not exactly a black and white answer...
CARDIO
First, let's look at cardio. Cardio workouts burn more calories while you're doing them. Your body weight and intensity will have a big effect on this (the more you weigh and the harder you work the more calories you'll burn). You'll burn a higher number of calories during a cardio workout then during a weight lifting workout for roughly the same amount of effort.
This is because cardio increases the heart rate resulting in a greater need for oxygen to move blood through the body faster. However, once your cardio sesh' is complete and your heart rate has recovered, the extra calorie burn ceases.
WEIGHT LIFTING
While weight lifting doesn't usually burn as many calories as cardio during a workout, it does increase your overall resting calorie burn by increasing muscle mass in the body. Muscle in the body requires more energy (calories) to maintain. So where a 150 lb woman might have burned 50 calories an hour while on the couch watching TV, by increasing her muscle mass by 10 lbs, she'll now burn 60 calories an hour doing the same thing.
While this might not sound like a huge increase in caloric expenditure, when considering her increased calorie burn for the whole day, over time, this can make some big changes.
Studies have also showed that calorie burn remained elevated for up to 38 hours after a weight lifting session. So while cardio burns more during the workout, the over all calorie burn can be much higher with weight lifting.
HIIT TRAINING
HIIT (High Intensity Interval Training) Workouts typically last between 10 and 30 minutes. These workouts include short bursts of high intensity exercise followed by low intensity recovery periods and have actually show to burn more calories that cardio while having similar effects in less time. This is beneficial if you are in a time crunch.
CONCLUSION
So, then should you be doing cardio or weight lifting? The answer is, you should be doing both. Cardio helps to burn a larger amount of calories, as does HIIT in a shorter amount of time, thus resulting in fat loss. And weight lifting helps to change body composition (build muscle). A combination of both will help you see the greatest results.