I assume the HSC you're referring to is High Step Challenge. Actually, HSTA is the harder of the two even though it doesn't have an ab section. It has tons of leg presses and some unique moves that are not in her other workouts such as one arm tricep dips, tricep extensions while doing static lunges, cross-over (walking side to side) pushups to name a few. HSTA is my favorite circuit workout of Cathe's. The cardio also gets your heart rate right up there. The cardio on HSTA has one minute more than HSC. Even though HSTA doesn't have abs, you are still working your core in HSTA. I have them both and love them both.
Marcy