Slow & Heavy because it is unique.
Pure Strength can be duplicated with a mash-up of Gym Styles, Max Intensity Strength, and additional other workouts, probably 4DS.
Slow & Heavy cannot be duplicated by any other in Cathe's workout library. It uses a very slow count, an 8 count, 4 to lift, 4 to lower, but you can play with this to accentuate either movement more according to your needs: I very often to 2 up and 6 to lower, could do 3 and 5.
S&H, unlike any other DVD Cathe has released, gives you time to think: form and to nail it. It is excellent to have and run through every now and then for this reason alone. The theory is: there's no momentum in the lift at all when the count is this slow, so you are forcing the muscle fibres to do more work, promoting growth.
I have never been so strong as coming off a 5-6 week Slow & Heavy routine. After that, anything seems possible! I am an ectomorph, hard gainer, and S&H works for me, like nothing else.
Clare