Is there a difference between the ToString method and casting to string?

There is a difference, yes. Every object has a ToString method, but not every object can be cast to a string.

int i = 10;
string s1 = i.ToString(); // OK
string s2 = (string)i;    // Compile error.

object o = 10;
string s3 = o.ToString(); // OK
string s4 = (string)o;    // Runtime error.

ToString() raises exception when the object is null, (string) conversion doesn't.