The slim series is for beginners to intermediate exercisers, P90X is very advance it focus is on muscle gain especially upper body, it is tough, but it is mostly weight workouts, you do cardio only twice a week, unless you have time to do doubles meaning you workout twice a day. IMO P90X is to gain muscle and definition, not to lose weight.
To lose weight the most important thing is the diet, a lot of cardio or circuit type workouts like, BC, or the terminators.