Slow and Heavy.
The Pure Strength Series is a fine series; however, the manner in which the reps are performed, the exercises used, and the abdominal work are fairly traditional, and you can find close-to-the-same drills in other productions.
The Slow and Heavy Series is quite different in that the tempos of the lifts are very, very controlled, and . . . well, slow, so this provides a very different training stimulus. Plus, it includes drills to strengthen forearms, something you don't see in other productions. And it incorporates a great deal of plank work and long-lever leg positioning in the ab / core routines.
I've seen most Cathe-ites use S&H for shorter, more intense rotations when they really want to build strength and mass. It takes a lot of concentration, and can be a bit of a time-chewer. But the reported results have been pretty significant.
JMHO -
A-Jock