Translate Mouse to Worldspace

I got a plane somewhere in the worldspce. I can detect if the plane is hitten by a mouseclick using Rays:

    if (Input.GetMouseButtonDown (0)) {
             Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
             RaycastHit hit;
             if (Physics.Raycast (ray, out hit)) {
                 GameObject x = hit.transform.gameObject;
                 if (x == gameObject) {
                 GameObject s =GameObject.createPrimitive(PrimitiveType.Sphere);
                     s.transform.position=hit.point;
                 } 
             }
         }

So when clicked again, I want to create a new Sphere where the mouse is, which this time havent to be on the plane. I know that I somehow need to translate my 2D coords in 3D. Since I now know the z coordinate this S shouldnt be a problem now or? I tried:

 Vector3 downpos=Input.mousePosition;
 downpos.z=s.z; // s from pervious code block
 downpos=Camera.main.ScreenToWorldPoint(downpos); 

But it didnt work. Why? And why isnt downpos equal to hit.point in this example?:

 if (Input.GetMouseButtonDown (0)) {
 Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 RaycastHit hit;
 if (Physics.Raycast (ray, out hit)) {
 Vector3 downpos=Input.mousePosition;
 downpos.z=hit.point.z;
 downpos = Camera.main.ScreenToWorldPoint(downpos);
 Debug.LogWarning(downpos+"?="+hit.point);
 }
 }

How can I make those two equal?

OffTopic: I would have used better tags if I was allowed to create them.