I have both, but haven't completed either yet.
Insanity is an awesome workout. Every time I finish one, I feel like I've just climbed Everest and am on top of the world. The hard part, is pushing play. I'm always a little scared to get started, lol. But I'm always glad I did.
I've only done the first workout of the p90x program so far. But I absolutely love it. I previously finished Chalean Extreme so I had some weight lifting experience prior to starting p90x. Let me just say I was sore for days after my first p90x workout! I'm a weird one and love being sore though.
Everyone Huffs and Puffs during Insanity. It's CRAZINESS! Modify any of the moves when needed and Saun T always encourages taking breaks when you need to! Don't feel bad if you take a lot of breaks, the goal is to workout to your max ability and as you get fitter and fitter you will take less and less breaks. Now insanity is definitely not for beginners (people who have been sedentary for months at a time) but if you've been active for a few months and are looking for a new challenge then I say go for it!