Problem comparing strings

As the title says Im having some problems comparing strings. At this point Im just trying to get a proof of concept so all I want to do is turn on an LED if a string contains another string. Ive tried a few ways now with no success so I though I would come here to get some advice. Ive tried declaring my strings in a few different ways to see if that would make a difference and Im still not getting any luck. What I have now is in the code below. I know that the subBuffer string will be at any point past the 2nd character, till any point till the end so I thought that using the substring command would work but it doesnt.

String Buffer;
String subBuffer;


if(Buffer.substring(2)==subBuffer)
 {
   digitalWrite(led, HIGH);
 }

Ive also tried using the strcmp but I also could not get that to work

Some simple code.

// zoomkat 8-6-10 serial I/O string test
// type a string in serial monitor. then send or enter
// for IDE 0019 and later

//A very simple example of sending a string of characters 
//from the serial monitor, capturing the individual 
//characters into a String, then evaluating the contents 
//of the String to possibly perform an action (on/off board LED).

int ledPin = 13;
String readString;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT); 
  Serial.println("serial on/off test 0021"); // so I can keep track
}

void loop() {

  while (Serial.available()) {
    delay(3);  
    char c = Serial.read();
    readString += c; 
  }

  if (readString.length() >0) {
    Serial.println(readString);

    if(readString.indexOf("on") >=0)
    {
      digitalWrite(ledPin, HIGH);
    }

    if(readString.indexOf("off") >=0)
    {
      digitalWrite(ledPin, LOW);
    }

    readString="";
  } 
}

Thanks for the help, Ill see if I can get it going!

String Buffer;
String subBuffer;


if(Buffer.substring(2)==subBuffer)
 {
   digitalWrite(led, HIGH);
 }

Buffer contains no characters. So, a substring consisting of the first two characters is undefined. Why would you expect that to match another empty string?