Well, of those 2 I 'd say Amy's ASC, but I really don't think that's her best step workout. I'd suggest ASC 2 or 3 instead. On both of them, she has a bunch of shorter premixes that would keep the workout shorter and the music is great. ASC had some production issues during filming and while it's a good workout, but you don't get the "full fun factor" of Amy. She also whoops a lot in it, something she doesn't really do much of in her later workouts.
I prefer Kelly's 30 min, Bootcamp and her 30 min Kickbox. The BC has 2 circuit workouts 30 min. long, but there's a 30 min. cardio only premix. The KBx one has 2 30-min workouts, w/ premixes that mix, match and combine the 2. I sometimes have a hard time w/ Kelly's cueing, especially for step. I nearly killed myself trying her Step Kickboxing 2! For BC and KBx, I can handle her cueing, but for step--no way.
JMHO and HTH!