Have to disagree with previous poster.
These two lower body workouts are radically different and I disagree that PS legs is more challenging. In many respects PS legs is easier. It may depend on whether you personally find strength work easier than endurance. I know I do. The previous poster is probably the reverse. Just because Cathe uses a 35 pound barbell throughout Leaner legs does not mean it is easier than the 40 pound bar she uses with the exercises in PS legs.
Leaner Legs is much faster paced, there are virtually no rests at all because Cathe uses the philosophy of constantly working opposing muscle groups so that you rest the quads while working haamstrings, but then she jumps right into the next exercise, there's no stopping. It maintains a great intensity upon the muscles and a 35 pound barbell is all you need. Where Cathe uses single sped reps throughout PS legs, with some of the counts held for more of an endurance feel, it is in Leaner Legs that Cathe first introduces "low ends," i.e. slow reps held at the lower end of the squat or lunge where you never come fully up, thus you never release the strain upon the muscle. The burn builds up fast, and a low end count of 7 can be excrutiating. This is, for me, much more difficult than the sit n' stands exehcise in PS legs, of which I can do several sets with a 50 and 45 pound barbell.
There is no floor work in Leaner Legs, this makes PS legs a good acquisition for your workout library. What makes Leaner Legs a necessary workout, I think, is that it will build your ability to do interval training and cardio plyometric work where you can withstand the burn in the legs of the interval or blast because you have tolerated the burn in endurance weight training. Leaner Legs helps me become way more efficient at Imax 2 and Circuit Max: it helps with my cardio endurance because it helps eliminate rapid onset of lower body fatigue.
I say, it is a workout well worth getting, in addition to PS legs. You will see how different hey are and feel, and you will enjoy good results with both. I know I always do.
Clare