Is it faster to cast or call ToString()?

I was wondering if I already knew an object was a string if it was faster to cast it or to call ToString().  I thought it would be faster to call ToString() because casting has to do some type checking while the String class overrides ToString to return itself:

public class String
{
     //...

     public override string ToString()
     {
          return this;
     }
}

By this reasoning, I figured that (a is string && a.ToString().Length < 4000) would be cheaper than (a is string && ((string)a).Length < 4000).  

It turns out they are both six IL instructions:  

Calling ToString  

L_0007: ldloc.0
L_0008: isinst string
L_000d: brfalse.s L_0023
L_000f: ldloc.0
L_0010: callvirt instance string [mscorlib]System.Object::ToString()
L_0015: callvirt instance int32 [mscorlib]System.String::get_Length() 

Casting  

L_0025: ldloc.0
L_0026: isinst string
L_002b: brfalse.s L_0041
L_002d: ldloc.0
L_002e: castclass string
L_0033: callvirt instance int32 [mscorlib]System.String::get_Length() 

So… now what is the cost difference between the castclass operator and the virtual method call to the ToString method?  

Turns out the cast takes about 75% of the time of a ToString call. 

using System;
static class Program
{
      static void Main()
      {
            object o = "a string";
            string a;
            string b;
 
            Boolean r = (o is string && o.ToString().Length < 4000);
            r = (o is string && ((string)o).Length < 4000);
 
            DateTime t = DateTime.Now;
            for (int i = int.MinValue; i < int.MaxValue; i++)
                  a = (string)o;
            Console.WriteLine(DateTime.Now - t);
 
            t = DateTime.Now;
            for (int i = int.MinValue; i < int.MaxValue; i++)
                  b = o.ToString();
            Console.WriteLine(DateTime.Now - t);
      }
}