I really liked her first series. I LOVE her step workout and while I've never done an Amy Bento step workout, I would compare Tracey's High Intensity Step Mix workout to Imax 2 in terms of intensity. The choreography, however, is more intricate than Cathe's and she sets up the intervals differently than Cathe does. Some of her interval moves are similar (squat thrusts, power scissors) and some are different (fast runs up on and off the ste, etc.) I think variety is great, so I really like this workout.
I don't like a lot of choreo when it comes to kickboxing so I actually like Tracey's High Intensity Kickbox Challenge. I use HIKC on a day when I am tired or looking for a mid range cardio workout (I'd gauge its intensity as being a bit lower than Powerstrike 3). I typically pair the cardio section of this workout with the cardio section from Cathe's CTX Kickboxing just to get in a 40 min. cardio workout. What I like most about her HIKC workout, like Karin, is the sculpting section. I only ever used to 'do' Cathe for sculpting/weight work, and the sculpt section of Tracey's kicboxing workout has some interesting moves that focus on balance and stability.
Your question was about cardio, but I'd like to give kudos to her Core Strength and Conditioning workout, which works the core very, very well. It's 45 long and it's given me DOMS every time.
Like Karin said, I think Tracey learned a lot from her first filming and as far as cardio goes, I think her new kickboxing workout (which also includes about a 20 min hi/lo workout) and her step workout are going to be fabulous.