Lizzy, you may just be right. It's just that I've had the stomach ever since I was around 10 and I've always wanted to get rid of it. I want my abs!
behaze, I can't really speak for the Insanity program just yet as I'm only about two weeks into it, but I did a full round of p90x I can definitely vouch for the effectiveness of the program.
Through the program, I lost about 30 pounds and 6 inches off my waist. I also had significant size losses all over, particularly around the legs area. Plus, add all the muscles I gained from the exercise and it's literally like looking at two different people. I have before/after p90x pictures which I'd be happy to upload If I can ever figure out how to do so on this site.
In terms of all out intensity, p90x is much more friendly towards someone worried about that. I consider myself in pretty decent shape now (an online calculator has me pegged at about 11% body fat, though the reliability of such methods is doubtful) but even I feel sore in the back and knee area after some of the Insanity exercises. P90X also shows ways you can do each exercise if it's too intense for you.
The biggest downside to p90x is that you need some equipment in order to do it. At a minimum, you're going to need dumbbells and a pull-up bar. This makes it kind of hard to exercise when you're away from the house/gym for the day. This is where Insanity really shines, as you can drop down and do the exercises pretty much anywhere you have a little space.
With all that being said, which one is better really depends on what your goals are. I feel p90x is a much more balanced out program, while Insanity is more targeted towards weight loss. After I finished p90x I still had a little fat I wanted to burn off, which is why I decided to go with Insanity. After Insanity I'm thinking I'll change to p90x+.
If you want more opinions, Beachbody has their own forum which has a huge number of enthusiastic users who would be happy to tell you why there program is the best.
Oh, and I didn't follow the nutrition guide. Even before I started p90x I had been doing extensive research on nutrition and had already developed my own diet, which I continued throughout the program.