These days, most workout programs incorporate both, often during the same workout. Most "cardio" work has an element of resistance training, either in the form of added weight or plyometric movements, while most "weight lifting" work has a cardio element because it's done in circuits.
There are two myths inherent in this age-old question. The first is that weight training will make you bulky. The second is that cardio doesn't build muscle. Let's dispel these once and for all.
Gaining bulk is hard. I wish I had a nickel for every time I've seen a hardcore gym rat who's been lifting for months, desperate for a few pounds of lean muscle mass, blow his stack when he hears a woman say, "Weight training will make me bulky."
It takes a ton of energy for your body to add muscle. During the initial stages of any kind of intense training, especially one you're not used to, your body releases excess amounts of the hormone cortisol, which causes your body to retain water. Some people think this means they are bulking up when, in reality, it's just the body adapting to the training. It happens whether you are trying to gain or lose weight and has nothing to do with gaining actual muscle mass. Once your body adapts to the new training, the cortisol release ceases and your body flushes the excess water.
Myth 2: Cardio Doesn't Build Muscle
This second myth is trickier. Low-level, steady-state aerobic training will atrophy muscle, so it can be true. But "cardio" hasn't meant aerobic zone training since Richard Simmons' heyday in the 80s. Cardio is a catchall term for any training that elevates your heart rate for the entire workout. These days, since almost all weight training is done circuit style, your heart rate remains elevated during both cardio and weight training workouts. Modern cardio training is almost always an offshoot of interval training, which means it's a mix of aerobic and anaerobic training. And this builds muscle.
Probably the easiest example to help you understand this is to look at some of the Beachbody®Success Stories. Take a look at the results from INSANITY® and those from P90X®. INSANITY is a "cardio" program that uses no equipment. P90X, on the other hand, requires you to lift weights every other day. Yet, the results you'll see from the two programs are remarkably similar. People tend to both lose weight and gain muscle. Adding muscle increases your metabolism and that's what helps you lose weight . . . as long as you're not eating too much.
What Is the Best Way to Lose Weight?The best way to lose weight is to follow a solid training system that targets weight loss. A system takes into account your entire lifestyle, workout, diet, sleep, and supplements. Why? Because all of these things affect your body's ability to change.
The key to weight loss is to change your metabolism. While it's easier to alter your metabolism through weight training than cardio, both will do it if the workouts are well designed. The word you're looking for to make this happen is intensity. By that, I mean you need to force your body to work in the anaerobic realm. Because your body depends on air to live, forcing it beyond its ability to breathe causes it to release performance-enhancing hormones to survive. When done consistently, these hormones change your metabolism.
Of course, you'll die if you stay in the anaerobic zone for too long, which is why you only do anaerobic work in short intervals. In between these intervals, your heart is working out aerobically to recovery. As long as the breaks between your anaerobic sets are strategic, you get a powerful cardio workout during every anaerobic workout. For example: circuit weight training—consisting of many sets to failure, with short breaks—is not only a great anaerobic weight training workout, but also a very effective workout for your cardiovascular system as well.
The next factor when it comes to boosting your metabolism and losing weight is recovering properly between workouts. This is why having a system is so important. Intense anaerobic training is stressful for your body. You need this stress to change your hormone balance, but if you overstress yourself, it will lead to problems in the form of overreaching and, if you do it too long, overtraining (both responsible for maladies from lack of results to injury or illness). A proper exercise program takes this into account by scheduling different styles of workouts next to each other to create a balance between intensity and recovery.
Your nutrition and lifestyle are very important for proper recovery. The better you eat, the faster you recover. Ditto for sleep. (I don't care how many episodes of Game of Thrones you need to catch up on.) Sleep is when your body produces its natural PEDs (performance-enhancing drugs). So get your shuteye!
All of this is why Beachbody never sells individual workouts that aren't attached to a program—or system—for your training. It's not that doing random exercise isn't good for you. It can be. However, we design workout systems that synergistically combine aerobic and anaerobic training (along with sundry other types of training, not to mention proper nutrition) to give you results. That way, it's much easier for your body to change its metabolism, and for you to lose weight.
To recap, whether your exercise routine is focused on cardio or weight training has very little to do with whether you'll lose weight. The best training programs have elements of both aerobic and anaerobic training and the important factors for weight loss are: pushing yourself beyond your comfort zone (aka intensity), eating enough to recover (but not too much), and resting enough between your workouts. Balance these factors correctly and your metabolism will shift and the pounds will melt away.