The directory script is not available, we are additonally about to drop it for another one.
Then about the differences between the phpBB SEO mod rewrite, the rewritten url will be very different.
Simple won't inject any title in urls, mixed will only inject forum title in urls and advanced will inject both forum and topic titles in urls.
If you are using phpBB3, give a try to our SEO premod
, you'll be able to test all the three mod rewrite at once, switching form on to another directly from the acp