title: Update value of one field based on another
body: <p>I have a template two text fields. A user creates an item based on this template and enters content into the first text field. </p>
  
  <p>On item save, I want to be able to manipulate that value, somehow, and write it to the second field when the item is saved.</p>
  
  <p>I have read about 3 ways of doing this - item:saved, item:saving or item save rules engine. I'm looking for an explanation in the difference of these approaches and if you had to choose which one would you pick?</p>