My Answer
P90X is generally the best all around fitness program you will find. It works out the whole body and seems to be best for overall slimming. It is about 65/35 Upper/Lower Body.Insanity is great for legs, speed, endurance, and agility. It is about 20/80 Upper/Lower body. If you have ever done P90X, it is easiest to describe it as Plyo XXX 6 days a week, with some Core Synergistics floor work thrown in.
The weakness of Insanity is that you wont see a lot of upper body gains from a strength perspective, unless you potentially add the v Upper Body Workout from the Insanity Deluxe edition. Also, as you go through P90X, you generally get stronger and feel better. This is primarily due to the yoga and stretch work and periodicity of P90X. Periodicity means that you are getting rest of your body parts throughout the program, so you can continue the program indefinitely. Insanity is pretty much all out for the length of the program, save a week in the middle of it, and you can get burned out towards the end.
Insanity however has its own benefits. Again, your leg strength and endurance will explode. The program is only 63 days vice 90, so you have a quicker deadline. Insanity workouts are generally shorter. You generally need to allocate 60-90 minutes a day with P90X, but only 30-60 minutes a day with Insanity.
While Insanity is definitely a full out exercise for most of the workout, P90X is definitely not easy. While full out circuit training, which is really what P90X is, provides periods of rest between sets, doing weightlifting sets to exhaustion should be as hard as Insanity. You just get the rest between sets.
The big potential drawback for Insanity is that it is non-forgiving, and can cause injury. Many people are forced to stop Insanity because of shin splints, knee pain, or similar issues. If you have good leg constitution, you will be ok.
