As TJ says, it absolutely depends on the chassis.
In my Italkart, “in general” the softer axle frees up the rear of the kart, harder axle gives more grip. That’s the opposite of OTK karts.
More often than not, most of us are best off sticking with the “baseline” axle, which is normally a medium stiffness axle on most karts, and looking to your driving or other smaller adjustments, unless you’re running in a big event on a very “rubbered in” / grippy track, or a very “green” / slick track.