Depends on your driving style and the track. Understeering is better
on fast tracks with sweeping corners, oversteer on slow tight corners.
Basically you want something in between, maybe just enough understeer
not to loose the tail in the fastest corner.
Soft suspension provides better grip, but less responsive handling
since the chassis is constantly adjusting itself. So basically you
have grip, but the chassis will rock from side to side and take away
the advantage (causes less precise control). Stiff suspension is
faster, but also harder to drive and can cause chassis upsets
(fishtailing coming into a fast corner e.g.).
Set the suspension as stiff as you can as long as it feels comfortable
to drive. For qualifying risk a lot stiffer setup for extra speed and
harder handling.