Key Differences

Resistance bands rely on the tension of the band to create resistance, while weights use the force of gravity. As a result, the resistance is from bands is constant, while the resistance of weights changes throughout the motion of an exercise. It also means that you can have resistance in multiple directions with bands, but with weights the resistance always pulls toward the ground. With weights it's possible to cheat, by relying on the momentum of the weight, but you cannot do this with bands.

P90X Recommendation

The "P90X Fitness Guide" does not recommend one option over the other. According to the guide you can use either dumbbells or resistance bands for any of the weight exercises in the P90X program. In fact, the P90X videos actually demonstrate all the exercises using both options. When performing the workout, simply follow the queues given by the demonstrator using the tool of your choosing.

Replacing Other Equipment

If you decide to use weights then you'll also need to invest in a pullup bar. But if you opt to use the bands for P90X you can go without a pullup bar. Several P90X workouts incorporate pullups, but according to the "P90X Fitness Guide" you can use resistance bands in place of a pullup bar. To do so you will need an attachment for fixing your band to a door frame. You will then mimic the pullup exercises by pulling down on the bands. This technique is demonstrated in the P90X videos.

Recording Workouts

When you do a P90X strength training workout, you should always record your workout using the worksheets included with the program. This allows you to track your progress and adjust future workouts. When using dumbbells you need to include the number of reps and the weight that you used. With resistance bands you should include the number of reps as well as the color of the band, which corresponds with the resistance level. Because you can also adjust the resistance level by adjusting the size of band loop, you should also note whether you use a small or large loop.