Navigon has nicer features (and is simpler to use), but IMO, Sygic has a more accurate routing algorithm -- ie: it gave me the same, accurate routes (more or less) that I would get from Google Maps, AT&T Navigator/Telenav and G-Map.
As stated above, my biggest concern with Navigon is that it has a tendency to send you on routes that are needlessly longer and more inefficent than necessary. That and the outdated POI database (which I found to be the worst of all the iphone GPS apps). On the plus side, it does have contacts integration, but I can all but confirm that both Sygic and AT&T Navigator are both working on contacts integration as well...