I am not Cathe, but I wanted to add my .02 (you might want to move this to the forum for discussing Cathe's workouts since the the people who post at this forum are very educated and informed and many have done all of Cathe's workouts).
Personally, I think the Blast Series is great -- you can adjust the music volume and the workouts are fun. My faves from this series are "Step Blast" and "Step, Jump and Pump!
However......
I also enjoy the Harcore Series....esp. the Gym Styles, Low Max, Muscle Max and the Core and Stretch Max videos. The music in Low Max is fantastic and it has the benefit of lower body shaping mixed with cardio.
The Hardcore series makes use of the resistance band whereas the Blast series does not. I think Cathe introduced the resistance band in the Hardcore series.
Both series have great premix options and you will not be bored, not matter which series you purchase.
If money is an issue, go with the Blast Series
It's a tough call -- I don't think you could go wrong with either one of these series.