There's a big difference between the two. P90 can be used by "beginners"...but don't assume that it's easy...just very straight forward and easy to modify for beginners vs. intermediate/advanced exercisers. P90 is broken up into 3 cardio days and 3 weight days with one day off. There is a level 1/2 cardio and weight routine and a level 3/4 cardio and weight routine. Your to do it for a 90 day cycle, beginning with level 1/2 and as you progress moving onto level 3/4. It's a good program, I've done it. I was in more of a beginner stage when I started it and progressed fairly quickly into the level 3/4 workouts. The workouts for each level are the same exercises, just pretty much more reps for the weighted work or longer sets of each cardio exercise.
P90X is more for the advanced exerciser. It's a series of 12 workouts that are pretty intense (some more than others...but overall not easy stuff). The upper body workouts are great and can produce AMAZING results. The X is pretty much Beachbody's most advanced routine. It wouldn't be something that you would want to just jump right into unless you're pretty acclimated to the higher intensity type workouts and have a pretty solid foundation of weight training. It's a great program and I highly recommend it if you're serious about working out.
If you're more on the beginner to intermediate level I'd say go for P90...but in all honesty when comparing Cathe to P90, Cathe produces stuff that is a lot better in my opinion. P90 can get a bit boring as you're doing the same exercises pretty much for the whole 90 days.
Hope that brief synopsis helps,
Deni
PS You can also go to the Beachbody message boards and read up about them or ask folks there their opinions on the two workouts.