I find Gym Style legs to be aimed more to endurance. The weights are light, for the most part, and Cathe plays with the tempo so you do a lot of pulses, etc, with just 2 * 8 pound weights for lunges, for example. This is so different from the 50 pound barbell static lunges in Slow & Heavy. Slow & Heavy does not play around with tempo. Each exercise comprises three sets of 8 reps at an 8 count per rep. This does not vary. You are encouraged to go as heavy as you can with good form and stay there for as long as you can. The feel is very different from endurance because there is simply no momentum helping you to lift that weight.
I think Gym Style legs is probably an excellent idea for most women, since we do tend to accumulate fat on hips, thighs and butt. I can't think that there are many women who want to bulk up their lower half. That's not to say that I never do S&H for the lower body, I do sometimes, then follow it with an endurance focus later in the week, or the next week. I like to keep changing it up for legs & glutes.
The S&H for the upper body is simply excellent. I am an ecto, with long skinny arms, small if wide shoulders and cannot do a single push up straight legged. With this series I have managed in 4 weeks to build some nice muscle and definition and have shed the skinny look for a sleek muscled look, with real bicep, tricep and shoulder definition. My back and chest are way stronger than they were. After these 4 weeks I am lifting heavier for the upper body than I ever have in my life before. I am happy!
I have not done the Gym Style for upper body so cannot comment on those yet. Perhaps someone else can chime in here?
Clare