Both are tough. P90x is more complete but insanity will kick your ass too. I'm sure many of the guys on this board laugh at but it will get you in incredible condition.
I'm a weight and running guy but having done body weight exercises for different periods of my life I know how hard it is. I'm sure a lot of these powerlifters would be very humbled quick trying to do an insanity, px90 or body weight routine.
I always amazed when in the summer I drive by a poor city in Asbury Park NJ. I see men doing chins, dips and other body weight exercises in a playgroud looking ripped. They could kick ass with the clients in the rich town over that has a fully equipped gym.
Look at how many wrestlers, boxers, elite military and MMA guys do only bodyweight exercises.
Also look up Ross Training for some really good books and dvds of hard core fitness.