I don't have Power 90, but I have P90X and SS.
P90X and SS are two very different types of workouts. SS focuses on using light or no weights and working the lower body every day (if you follow the system) in order to stay lean. The problem with this is that if you follow their rotation, you are also working the upper body every day, which I found to be overkill on the joints, even with light weights.
SS workouts are very similar: simple moves like squats, lunges, step touches, some isolated kickboxing moves (they have bad form!). No moves linked together in a choreographed sequence. There is a "music off" option so you can use your own music.
Some people say that SS workouts are boring, but sometimes they are the only things I can get myself too do. I put on my own tunes (anything from 130 to 136 bpm or so works) and follow that beat. I don't have to think about choreography.
All SS workouts contain leg work (even "Cool it Off," the recovery/stretch workout, has some knee stablization exercises), and one, "Firm it Up," is all lower body.
P90X is my best fitness purchase for 2004, and perhaps of all time. I found the upper body work to be without compare as far as results. I needed different lower body to keep my glutes where and how I like them to be! Cardio is minimal, but the goal of this program is building strength and muscle tone. I highly recommend P90X to anyone who wants to go a step beyond Cathe's workouts (but add in Gym Style Legs once in a while instead of the P90X legs). The legs are worked in "Legs and back' (the only focused leg workout in P90X), but also in the yoga workout and the plyometrics workout.
I don't have Power 90, but see no appeal in it. It's several steps below P90X, and seems more repetitive (12 or 13 workouts in P90X vs 4--I think--in Power 90).
If you want an cheaper, abbreviated taste of P90X (with Tony goofier than he is in P90X), Power Half Hour is a set of 5, half-hour workouts on one DVD. The sequencing is similar to P90X weight work (push-pull) and the abs workout contains some plyo moves like in PlyoX.