Add some fileds to the wp_posts table

The second approach (joining additional table) is better for several reasons.

• When plugin is not active, you won’t retrieve it’s data (or with other words, you won’t select additional data unless you explicitly join it)
• It’s more maintainable. You can alter your plugin table without altering posts table and vice versa.
• The join most likely won’t hurt performance, but may improve it if there are a lot of updates – (one more post on the same topic)