What is the difference between the Drupal 6 and Drupal 5 Versions of the Parsing API?

Absolutely nothing. The only difference is the .info file. Actually, there is 1 other difference. At the top of the D6 version there is a comment called //v6 just so I know which version I'm working on. So this means that this module can actually be used with any drupal version (4.x, 5.x, 6.x, 7.x, 1843333.x) because it relies on no drupal functions at all. It is 100% pure PHP.