It depends.

Assuming that you are already in good enough physical shape to take up the P90X or Insanity program, choosing which one to purchase depends on your individual fitness goals.  Do you want to tone your overall body and increase your strength?  Or, is your primary goal to maximize your cardiovascular power and stamina?  Your answer to these basic questions will tell you which program to buy.

I want to tone my overall body and increase my strength.

If the above statement applies to you, then you will want to buy the P90X program.  Although P90X will increase your cardiovascular fitness a great deal, the program will have more benefits for those wanting to increase their body mass, muscle strength, and muscle tone.  Most of the workouts in the program focus on particular body areas.  For example, my favorite workout focuses on shoulders, biceps, and triceps.  When I do P90X I switch between upper body and lower body workouts so that my entire body gets worked out by the end of each week.  The result?  I begin to notice muscles that I did not even know existed!

I want to maximize my cardiovascular power and stamina.

If the above applies to you, then you will greatly benefit from purchasing the Insanity program.  This program will increase muscle strength and muscle tone, but not as much as P90X.  For example, Insanity includes only one upper body workout for strengthening and toning the shoulders, chest, and arms.  The rest of the Insanity program consists of extreme cardio workouts aimed at helping people reach their maximum fitness potential.  I am completely wiped out by the end of each workout no matter how physically fit I may be at the time!  With Insanity my overall body mass significantly decreases, meaning that my muscles shrink.  However, my cardio performance skyrockets.

Which challenge will you take?

Both programs will challenge you to reach your maximum potential.  One program is not necessarily better than the other, but you need to recognize your  fitness goals before buying P90X or Insanity.  To recapitulate, P90X will greatly help you strengthen and tone your entire body.  On the other hand, Insanity will whip you into shape with calorie burning cardio workouts.  I highly recommend both exercise programs; however, you will get the best results by purchasing the program that complements your personal fitness goals.