I'm just finishing up week 9 of P90X and I do love it; however, I MISS CATHE!! Prior to starting the X, I did a round of Insanity and prior to that I did mostly hardcore Cathe stuff. Cathe got me in the best shape of my life and then I got amazing results with Insanity when it came to muscle definition and cardio endurance, but no weight loss (not sure I really had much to lose). I AM getting a lot stronger with P90X, but since I was in such good shape to start with, my results have not been very dramatic. I would LOVE, LOVE, LOVE to do STS, but right now the biggest thing holding me back is....THE COST. I have had to add in HiiT training with my P90X strength days because the program in itself is too slow paced, but I'm pretty sure I would have to do that with STS too. One of the most appealing aspects of STS is that you get to do a different DVD everyday! With the X, it's essentially the same 8 or 9 workouts over and over, but like someone else said, it has been proven countless times that it works. So, due to cost, I chose to go with P90X, but am saving to purchase STS someday. Anyway, so sum up this post, that is all over the place, since you've done STS, I think you would like P90X. Yes they are very different programs, but both work on the concept of muscle confusion so in that regard, they are very similar.