TextNode or textContent?

It ‘s not really matter of advantage but of proper usage depending on the need.

The fundamental difference is that:

• createTextNode() is a method and works just as its name says: it creates an element… then you must do something with it (like in your example, where you append it as a child);
  so it is useful if you want to have a new element and place it somewhere
• textContent is a property you may get or set, with a unique statement and nothing else;
  so it is useful when you only want to change the content of an already existing element

Now in the precise case of your question, you said you want to change the text of the element…
To be more clear say you have the following HTML element:

<span>Original text</span>

If you’re using your first solution:

var my_text = document.createTextNode('Hello!');
span.appendChild(my_text);

then it will end with:

<span>Original textHello!</span>

because you appended your textNode.

So you should use the second solution.