I don't know, my brother and I are both Tony Horton marks since we used his Power 90 (kind of the precursor to P90X) to get into shape. Well, me more than my brother. I do have one disc of P90X+ and I can say those two routines (just abs and upper body) are fucking killer. Really, I think you'll get decent results with either. Going off bcsaint's comment, consider how much time you can dedicate to a workout or you want to dedicate as well as how much you can afford. I think Insanity doesn't require any equipment, so that's one advantage in terms of cost. In P90X, they want you to have resistance bands or dumbbells, push-up stands and Beach Body's multi-positional pull-up bar. So yeah, a lot more shit for which to shell out the bucks.

Depends on what you want
Insanity focuses almost solely on cardio
P90x focuses on changing the workout and exercising different areas of the body.

From personal experience, I would say that P90x does provide better results though.

Insanity or P90X if you are contemplating suicide.