I hear P90 is more mellow and better for beginners while P90X is for the more advanced exerciser. I have never done P90 but hear it is boring and "too easy". I have done P90X and it is advanced. If you are doing Cathe now, I'd say go for P90X, P90 will most likely disappoint you.
When I did P90X, I did work Cathe in. I did her GS Legs weekly since the P90X leg work was not intense enough.
If you do a search for "P90X" on this forum, you will get a ton of threads with tons of information.