I have tried both.
Don't try any of them if you have bad or weak joints!!! (its all explosive compound exercises)
What i will say is it takes a week ish to get used to the exercises to be able to do them properly without having to look at the screen or workout what position to get into next. Once you know the exercises and sequences it becomes a lot easier.
The Insanity uses your own body weight as the resistance whereas P90X uses things like a bull up bar, weights, bands or medicine balls and is no use without them.
I usually make it to about week 2, mainly because i get bored (its just not my method of training) but those that stick with it usually do it month after month.
Go for Insanity unless you want to spend as much again on equipment.
Have a look on Google Videos, i'm sure there are quick snippits of the training and some before and after type videos, you can use this info to make your choice.