HttpRuntime Cache vs. static dictionary/fields

What are the main pros and cons for using HttpRuntime Cache against using simple static field?

I need to store data in scope of entire ASP.NET application.

HttpRuntime.Cache["MyData"] = someHashtable;

vs.

private static System.Collections.Hashtable _myData;
public static System.Collections.Hashtable MyData
{
    get
    {
        if (_myData == null)
        {
            _myData = new System.Collections.Hashtable();
            // TODO: Load data
        }
        return _myData;
    }
}