They are very different, and complement each other, IMO. P90X stresses muscular strength and growth, with minimal cardio to not burn up the muscles. It is also a very complete system, with stretching, yoga, plyometrics, gym-style (actual gym style!) weight work, etc. The lower body workout uses light or no weights, and I found that it did tone my legs, but wasn't enough for my glutes/hamstring area. Next time I do P90X, I plan to alternate between the P90X leg workout and something like GSL, which I think would be a great addition to the program.
Slim series workouts are very similar to each other, and are boring if you follow their rotation (doing an SS workout every day). They are all based on using light or no weight, and doing many reps. It's particularly good, IMO, for keeping the lower body slim. The stretch workout (which is more than stretching, more of a "recovery" as it includes some knee stabilization and some spinal extensor work) is excellent! The choreography (if you can even call it that!~) is minimal: very simple isolated moves. I sometimes can't bring myself to do anything BUT this kind of workout. I don't have to think about it, I can use my own music ( there is a "music off" option on all the workouts, as there is with P90X), and I follow the beat of the music and do whatever they are doing for the amount of time they are doing it.
To me, P90X is one of the best fitness investments I've ever made. SS is nice to have around, and I do go back to it regularly. Except for "Firm it up" (all lower body work), I have a hard time keeping the workouts straight!