title: ngModel vs keyup/down
body: <p>while learning angular2, came across these two ways to read value from input field as below,</p>
  
  <pre><code>&lt;input type="text"[(ngModel)]="pname"&gt; // I can read the value from --&gt; pname
  &lt;input type="text" (keyup)="addvalue($event)"&gt; // I can check for enter press and read text
  </code></pre>
  
  <p>For ex, if I want to write a ToDo app and ask user to enter the list in input field, post clicking <code>Add</code> button, could add it in list</p>
  
  <p><code>keyup</code> will trigger the method for each keypress, won't it affect the performance?</p>
  
  <p>please explain the scenarios when to use <code>[(ngModel)]</code> and <code>keyup</code></p>