I've wondered this many a time! The only differences are that one of the XTrain legs workouts is also cardio (Cardio Leg Blast) and there is a metabolic weight training dvd in Xtrain (Supercuts). Of course STS has Plyo legs, but only in Meso#3.
One reason I think I achieved more muscle growth in STS was that I progressed my 1RM much more. I think, because the training is at a slower pace, I felt more comfortable with constantly increasing my weights.
Hopefully others will chime in with their experiences.