I think the degree of difference between the two will vary from country to country as the Navteq map is better than Tele Atlas in some countries but worst in other countries.
I'm speaking from my experience in Australia only.
I first bought Sygic, and it is so good that I said in many forums I don't need TomTom anymore.
When Navigon was released, I looked at the reality view, the iPhone keyboard, the map zooming, and bought it right away, and declared that I'm dumping Sygic for Navigon. I was wrong. It Navigon turned out to be so bad that I had to return it. It has no speed limit for all the Sydney Metropolitan roads that I tested it on. No speed camera warning, No red light camera warning. No school zone warning. None of the intersection I travelled through had reality view. POI is very limited. GPS reception is weak compared to Sygic's. Terrible choice of text color and font size making the road names hard to read. You need to touch the top and bottom bar to rotate through the key navigation information, hence making using it more dangerous. Not able to simulate the entire trip.
I had since got a refund for Navigon, and have bought TomTom.
I've been playing around with it for the last few hours and driving around with it. This is my first impressions:
Disappointments:
1. No abiliity to warn that you're approaching a school. This surprised me, and it had always been possible in my winmo TomTom, so I had assumed that it would be the same. Not so. Now I feel that I no longer can rely on it to remind me to check if it is school hours and if so slow down to 40kph.
2. There is no audio warning for overspeeding!!!!! WTF! Only visual warning is available. This is not what the product description says. It says there would be both visual and audio warning. Is this a bug? Is there a hidden setting that I have failed to switch on?
3. So far, I have seen no evidence of a lane assistant appearing yet. Perhaps more test is needed to confirm this. Anyone has a view on this after testing?
Positives:
1. Best user interface I have seen.
2. Road speed limit as complete as Sygic's. However, the red light database does not include one red light near by house, whereas Sygic's does.
3. It differentiates between red light and speed camera and shows a different sign on the screen; whereas Sygic shows the same icons for both. I would have prefered it using a different sound for audio warning, but TT iPhone seems to be using the same warning sound for both types of cameras.
4. Excellent trip simulation and advanced planning. Have not tried planning for multiple destinations yet though.
5. I'm able to use it without setting a destination. The speed monitoring, red light/speed camera warning all works as if I'm having a destination. Of course, the overspeeding warning is still only visual, with no audio warning.
When I did my first run, for some reason the cursor position kept jumping about and failed to stick to the road. It also seems to lose its orientation. The strange thing is this never happen again when I did the subsequent tests.
Now to be honest, I'm quite disappointed. Perhaps my expectation for TomTom was high, especially when the price is so expensive. If only Sygic uses the standard keyboard and let us zoom the map the standard way, I would stay with Sygic. Now I'm torn.
and can do multitasking... 
