Use raytraced shadows.Those will give u sharp edged shadows.Now the trick to making them softer is to use the area light option in your light attributes.U can choose Sphere, cube etc.Scale the respected choice (sphere, cube)according to your scene.Play around with the samples settings within the area light options.Increasing the samples = increasing render times.Overall using the area light within the mental ray light will dramatically increase render time.Try it out.