1. The displacement of a spring is proportional to the force applied to the spring. The potential energy stored in the spring scales with the square of the displacement, and therefore the square of the force. A soft spring will therefore store more potential energy than a stiff spring when an equal force is applied.
2. All real springs are damped, so some energy is lost when a spring is compressed and then allowed to relax. Given two springs, one stiff and one soft, with equal losses, the soft spring will lose more energy than the stiff spring when subject to the same forces.