variable.ToString() vs. Convert.ToString(variable)

int someValue = 4;

// You can do this
txtSomeValue.Text = someValue.ToString();

// Or this...
txtSomeValue.Text = Convert.ToString(someValue);

//This will set the variable test to null:
string test = Convert.ToString(ConfigurationSettings.AppSettings["Missing.Value"]);

//This will throw an exception:
string test = ConfigurationSettings.AppSettings["Missing.Value"].ToString();