Navigon is a better product overall, especially when you consider the amount of money they are charging. TomTom is better at calculating initial routes and calculating alternatives, but it does not have traffic information yet, so the advantage of this is a bit limited depending on when/where you drive.
Navigon has nicer features - the map is better-looking, the lane assist/reality view is better, the text-to-speech is extremely good, and it only gives you the information you need. For example, some highways might be an interstate plus a few U.S. or state routes, but Navigon will just tell you what sign you need to look for, not the name of all of these routes. TomTom tells you everything, so by the time it's done spitting out all of the text, you might be past the turn. I just use one of the non-Computer voices when I use TomTom.
The problem is that Navigon is unusable on a 3G in my opinion. The text-to-speech cuts out all of the time, and usually will stop completely at some point. The other day, the TTS actually never started for me. It's really bad, and the iPod playback is really choppy and distracting, making it useless.
At this point I wouldn't recommend either. If you put a gun to my head, I'd pick TomTom now for the 3G. Navigon did nothing but frustrate me with its sluggishness.
