P90X is a high-intensity home workout program that combines aerobic and anaerobic exercise with yoga, core exercises and stretching. The program is designed to dramatically increase muscle tone and fitness in 90 days. In order to achieve those goals, you'll need some sort of resistance equipment -- free weights or resistance bands. Both have their advantages and disadvantages.
Advantages of Weights
Accuracy is the major advantage free weights have over resistance bands. With dumbbells, you know exactly how much you're lifting. This enables you to track your progress much more accurately than with bands.
Bands don't provide a weight equivalent. They come in different levels of resistance, which means you can progress as you get stronger, but they are much more of an estimation of progress than free weights. If your goal is to build muscle mass or you have a particular strength goal, you may get better results with free weights.
Disadvantages of Weights
Weights are not portable. If you are traveling, you cannot tuck them into your suitcase as you can do with bands. They're also hard to store and will take up more space in your home.
The biggest disadvantage of weights is the cost. A good set of free weights can cost several hundred dollars. If you want to progressively overload to build mass, you need a good variety of weights and potentially some very heavy weights.
The risk of injury is higher with free weights, especially if you are using very heavy weights. It's easier to strain a muscle lifting free weights with poor form or injure your foot by dropping a weight on it. Lastly, if you drop a heavy weight you can damage your floors or other items in your home.
Advantages of Bands
Resistance bands travel and store well. They are light and portable, and you can just tuck them away when you are not using them. Resistance bands are much cheaper than free weights. A set of resistance bands is a fraction of the cost of a set of weights.
Resistance bands provide versatility in movement that free weights do not always provide. Because elastic bands don't rely on gravity for providing the resistance, you can use them to provide gravity in the horizontal plane, such as twisting or punching moves.
Disadvantages of Bands
You must learn how to use resistance bands, and they are not as intuitive to use as weights. If you have always used weights, you won't be able to just mimic the same movement holding a resistance band. You have to understand how the band creates resistance and how to hold or anchor the band to get that resistance in the right place.
Bands are not as precise in terms of tracking and logging your progress. If you adjust your stance or "choke up" on the band you can increase the resistance, but you do not know by how much. And, you may have difficulty maintaining this exact level of resistance on your next workout.
Inexpensive bands often break or snap. A band that snaps can cause injury. Buying high-quality resistance bands and taking good care of them can prevent this, but it is still a risk.
The Bottom Line
Decide what your goals are and how much is in your budget. If you know you're going to use free weights for a long time, they can be a good investment, as long as you have somewhere to keep them. If you're a beginner or dabbler in fitness, making a big investment in free weights may not be prudent right now. Do the program with resistance bands; you can always purchase free weights for your next go-round.