I completed the Insanity program and lost 20 lbs while on it. With that being said, Insanity and p90x yield similar results, but cater to specific needs.
p90x is mostly about strength gains, while insanity is about HIIT training to blast away fat.
Which one is better? Either one works, it just depends on what you want. I was 181 at 5'6, and now I am 154. I wanted to get rid of the fat by dieting + insanity. Only reason why I did it was because I couldn't do a pull up for the life of me, so p90x wasn't that appealing to me.
After I incorporated the gym + insanity in my regimen, I can now do 15 pull-ups straight. It's not much compared to others, but going from 2 pullups to 15 as of current is a good personal achievement.