To answer your question soft is better for grip, stiff is better for chassis response.
At the end of the day though, how you set up your car is also going to depend on your tire compound choice.
A softer suspension is better with less grippy tires: If you go too stiff especially on comfort/sports hard tires, the rate at which you understeer OR oversteer is increased. What that means is you won't be as consistent driving fast because you'll be caught off guard with what the car is doing real quick. A softer setup with less grippy tires will ensure you approach the limits of the car progressively and making it more forgiving.
A stiffer setup is better with grippy tires: The car grips more with stickier tires so it will lean/squat/dive more when taking a turn/accelerating/braking. I personally don't like it to lean too much so i'll compensate by increasing just the spring rate and maybe the compression.
