ReactJS state vs prop

Your second approach is more like it. React doesn't care about models so much as it cares about values and how they flow through your app. Ideally, your post model would be stored in a single component at the root. You then create child components that each consume parts of the model.

You can pass callbacks down to the children that need to modify your data, and call them from the child component.

Modifying this.props or this.state directly is not a good idea, because React will not be able to pick up on the changes. That's because React does a shallow comparison of your post prop to determine if it has changed.

I made this jsfiddle to show how data could flow from an outer to an inner component.

The handleClick method shows 3 ways to (im)properly update state:

var Outer = React.createClass({

  getInitialState: function() {
    return {data: {value: 'at first, it works'}};
  },

  handleClick: function () {

    // 1. This doesn't work, render is not triggered.
    // Never set state directly because the updated values
    // can still be read, which can lead to unexpected behavior.

    this.state.data.value = 'but React will never know!';

    // 2. This works, because we use setState

    var newData = {value: 'it works 2'};
    this.setState({data: newData});

    // 3. Alternatively you can use React's immutability helpers
    // to update more complex models.
    // Read more: http://facebook.github.io/react/docs/update.html

    var newState = React.addons.update(this.state, {
      data: {value: {$set: 'it works'}}
    });
    this.setState(newState);
 },

  render: function() {
      return <Inner data={this.state.data} handleClick={this.handleClick} />;
  }
});

Updating 2016: React is changed, and explanation "props vs state" became very simple. If a component needs to change data - put it in a state, otherwise in props. Because props are read-only now.

What's the exact difference between props and state?

You can find good explanation here (full version)