I would say that STS is more like P90X, only much more varied--a big plus in my opinion.

Chalean Extreme seemed to focus on compound movements. Meso 1 in STS moves quickly from one exercise to another. In Meso 2 and Meso 3, you really take your time with each exercise and muscle group.
STS Cardio was designed to go with the STS weight program, which is why some of the cardio (MMA Kickbox for example) is much less intense than others (HiiT or CCC). I love the way STS Cardio fits with the STS weight program. But you can use any cardio with STS.