Ive done both. P90x is more weight training. It may be difficult but is great. Also with instanity it is more interval training.Where you are doing alot of weight bearing exercises and jumps etc. That may be more difficult to start with if you have a hard time with endurance. YOu can do anything you want to if you put your mind to doing it no matter how hard it is... You will see results on both programs.
'Fat' people would be able to do some exercise, but will not experience the dramatic results as shown on teambeachbody.com. I was already in shape from snowboarding, mountainbiking, running, kayaking, and it (insanity) still kicked my butt the first week. People completely out of shape would have to do the program once just to be able to get the exercises and movements right so they could begin a second time to actually do it correctly. I could see someone getting injured trying to keep up if they don't listen to Sean T's advice. I want to try P90X. Each DVD looks like it is for a different muscle region. Workouts are longer too. I like the short duration of insanity workouts and the fact that it ruins (in a good way) your whole body every day.