Regular commenter and tipster poedgirl actually forked out the coin for both Australian navigation iPhone apps currently available on the App Store. Then she reviewed them. And the results aren't quite what I expected...
From poedgirl's review, the Sygic app seems to wipe the floor with Navigon, scraping it's face along the rough bits before dumping what's left in the nearest waste facility. Everything, from the user interface to the quality of the WhereIs maps over the Navigon's Navteq ones all seem to end up in favour of the Sygic app.
Little things, like being able to manually set your Home address, are made unnecessarily more difficult by the Navigon app, while the Navteq maps seemed to have a few errors in poedgirl's testing. Although one of her complaints - that the Navteq maps said her house number didn't exist - is a problem I face with every mapping software, and is only a minor quibble.
Still, things like user interface are incredibly important for a navigation app, and Navigon's apparent lack of customisation options seems to hurt it.
If you're thinking about picking up either app, the review is definitely worth a read.