I have both and am on my second rotation of STS. I started P90X but then DH decided he wanted to start working out so I got him bands (he travels) and gave him my P90X. While doing it, though, I loved it and I thought Tony was great. I did not, however, do anything but weights and did Plyo X once, which was a lot of fun. My plan was to alternate weeks of STS and P90X, keeping the reps/weights of P90X consistent with the mesocycles of STS (endurance, hypertrophy, strength). This would have basically been like doing the 6 month rotation of STS but with great variety. I liked the moves in P90X for a change. It seemed that STS is harder though, weightwise, but if you combine P90X with the other X workouts, they may even out more. Can't say for sure. I do know people who have gotten great results from both programs.