Softer=less tire wear. With stiffer springs your car generates less traction with the suspension and relies on the tires more. Keep in mind that you can go too soft and have a car that wallows around too much.
What I run: Schumacher Mi5/Associated RC10R5.1/Associated RC12R5.2/Futaba/HobbyWing/Team EA Motorsports/BSR Racing
Where I run: Florida Indoor R/C Complex/Thunder Racing/Florida On Road State Series