IMO, Navigon is designed more like an iOS program. TomTom makes use of iOS design paradigms, but mostly it reminds me of their stand-alone products. Lots and lots of menu items which can get a bit confusing.

I've had a quick look at both and they're each quite good in their current incarnations, at least while stationary. TomTom feels a little slower when browsing the map. I like the loupe in TomTom's 2D map browser - it makes it precise to drop the destination exactly where you want it and lets you identify a specific POI. Navigon sort of drops the destination onto whatever is closest to the middle of the screen, however it has logos to identify the different popular POI brands. Funnily enough, it's missing McDonald's while that chain does appear in TomTom. I was just testing with that chain - I've eaten that food once in the past 5 or 6 years.

I do wish that both would do a little more offscreen rendering so that when you scroll the map it's not so slow. None of them is going to win any beauty contests against the bitmap images produced by Google Maps - I especially like Google's shaded representation of major buildings.