I'd like to put longer springs on my Vitara   (european Tracker) in future. So that will mean a suspension lift. The question now is what's better for off road use Stiffer or Softer springs. My idea is softer because its easier to flex then, am I right

Pitch of the spring is important, more coils = lower pitch and much "softer" but with the right rate they will support load fine.. My 97 Vit has 3" lift and the rear springs are approx 19 " long when laying on the ground  and have 13 coils. I never thought they would fit !! No problem.. the fronts were a full two coils longer than std..  Off road its very flexible and thats what u need !