Slow & Heavy is for pure muscle building. It has the fewest amount of reps so you can go as heavy as possible. Gym Styles has more reps so it is still muscle building but has some of an endurance component as well. The Gym Styles also use a resistance band. Cathe uses this for a "bonus" burn. They are both great series. Also, Gym Styles does not have any core work since Cathe made Core Max which has 3 great core routines plus 3 premixes.
Marcy