I also use KAP Challenger Crafts and am really looking forward to the day when I can do justice to a 'better' set of limbs.
My heart says Borders. Principally because of everything I've heard and read about them but also because they are made reasonably locally to me and I appreciate local craftsmen and women.
Having said that, if they don't suit me when I try them, I'll be very disappointed but will look elsewhere.
The KAPs are more than good enouogh for me at the moment and will, I'm sure, remain so for some time to come.