React Testing Confusion

When to use testutils functions like

to test components and when to use refs?

say we have parent Parent component and inside that is Child child component

const parent = renderIntoDocument(
         <Parent />

Now if we want to select Child child component I think, we have two options-

1. const child = scryRenderedComponentsWithType(parent, 'Child');
   const el = ReactDOM.findDOMNode(child[0]);

  2.  const child = voting.refs.someref;  ("Assuming we have a ref element on child component with value")
      const el = ReactDOM.findDOMNode(child)

My question is when to use first one and when to use second one and how to decide? Because in different situations both are not working same

Method 2 looks cleaner. No idea when you would use each. I have little experience with React.