Both shafts were designed by the same person, Kim Braly (the KB of KBS). PX was designed to be a boardy (firm) feeling shaft with a lot of stability, originally just as a prototype for Phil Mick, but it started to get used by other players and eventually came to retail. PX has a medium launch but a flat flight, so the ball does not balloon. KBS is intended to have a similar flight, but also to add some feel.
If you demo them side by side, you may por may not notice much difference, and it is hard to say based on the technology which you would prefer, so there is really no substitute for demoing both.
I have 2 sets, one of each, and like both a lot. The KBS are in my main set, so I have a slight preference for them.
Thanks ethan. I think it's worth looking at view had a set of mizuno with the px shafts but think I'll hang fire for a bit longer when I try them w
ith kb. s shafts