What is the difference between using the StringBuilder class in Java's toString() method and using the string connection operator?

public String toString(){
    return "{a:"+ a + ", b:" + b + ", c: " + c +"}";
}

public String toString(){
    StringBuilder sb = new StringBuilder(100);
    return sb.append("{a:").append(a)
          .append(", b:").append(b)
          .append(", c:").append(c)
          .append("}")
          .toString();
}