I think both are must-haves. They're very different workouts. KPC is longer & higher impact. The first half consists of linear drills (basically a single high intensity KB move done something like 16 times on each side), the second half consists of 3 choreographed KB combos that you build up to.
Strike Zone is a bit shorter, lower impact, & consists of (I think) 7 sections ("strikes") of short drills. There are no combos, it's completely linear.
I'm liking SZ more right now b/c it's new for me & lord knows I've done KPC maybe a zillion times over the years, but I do love KPC & probably pop in that DVD once a month or so.
Jeez, just buy them both already! :+