The short answer… both
Sprinting is one of the most powerful and natural movement patterns which synergistically builds muscle, bone and connective tissue strength in the foot, lower leg, upper leg and hips whilst also providing a great cardiovascular workout. As a complete exercise for improving your strength, fitness and physique few things can compete.
To further increase the workout demands (strength and cardio) try sprinting up a hill or gradient. Counterintuitively this makes a sprinting session both harder and safer as the hill applies a natural set of brakes to your body and dissipates some of the impact forces.
For a combination of strength and cardio benefits you can’t do much better than sprinting.