Global variable Vs ViewState performance.?

Hi Everybody,

I have one requirement, I got confused which one is good in this scenario. Please help me out of this.

I have a QueryString value, I want to use this in through out my application for this I follow the below options.

Option 1:

Declare Global variable and assign QueryString value on load event of that page and use that Global Variable whenever I need.

EX:

string Id="";

protected void Page_Load(object sender, EventArgs e)
{
    if(Request.QueryString["Id"]!=null)
        Id=Request.QueryString["Id"].ToString();

    if(!IsPostBack)
    {
        //pass that Id and do whatever I need on here.
    }
}

Option 2:

Store ViewState inside of IsPostBack and use that viewstate value whenever I need that.

EX:

protected void Page_Load(object sender, EventArgs e)
{
    if(!IsPostBack)
    {
        if(Request.QueryString["Id"]!=null)
            ViewState["Id"]= Request.QueryString["Id"].ToString();

        //perform my action based on Viewstate value.
    }
}

In option-1, I just declare one global variable and assign value on load event and use that value whenever I need.

In option-2, I just assign querystring value at once and store it into one viewstate and use that viewstate value I can implement my task.

Now, my question is when we go with Performance which option is good.