CK is a so so workout. The music is rather bla and the routine is repetitious. In my opinion, KPC is Cathe's best kickbox workout followed by Kickmax (although I don't love this workout). Cardio kicks is ok. Her form isn't so good and there are a lot of moves that you would never do in a true kickbox class (I did boxing/kickingboxing in a kickbox only gym for 5 years). For me, and only for me, I don't find Cardio Kicks to be that difficult (keep in mind though that IMAX 1 is something I do when I don't feel like working out very hard. I am a very different species compared to everyone else).
Aside from all of my personal opions, have fun no matter which workout you choose. It is a very individual process.
Good luck,
Rhonda