I have used both, Ovi on my N95, Sygic on my N900.

I can give you my opinion but, as with all opinions, this is subjective...

Interface: Maybe not a fair comparison as this is more hardware dependent but Sygic just shades it for me. Ovi is simpler to set up prior to setting off but Sygic is easier on the fly. It is a bit quirky at times but once you're familiar, it's fairly straightforward.

GPS: Sygic is quicker to lock, especially when moving but again, probably more to do with hardware. Sygic arrow lags behind actual position by about 20m (I know there is a mod that can help) but again not a problem for me as I quite like seeing the turn ahead rather than being on top of it....

Routing: I've had some "interesting" journeys with both but have always reached my destination which is really all that matters to me. Don't really use it for known routes so woudn't know if it took me the long / wrong way....Sygic's roundabout handling can be a bit confusing 'go straight over the roundabout' is used for just about every exit....

POI: Sygic seems to be missing some rather big ones.....

Voice Guidance: Sygic could be louder through the N900 but I plug into car speakers so not an issue for me. Preferred the female voice of the Ovi over the male Sygic but not a deal breaker.

Stability: I have found Sygic more stable but again could be more hardware based as I found my N95 used to lock up on complicated routes.

I drove from Scotland through and around France and back again with my N95 and it was perfect. N900 has only been used around the UK but no problems so far.

I keep my N95 in the car as backup but have never had to use it.

Both do the job and, reading my own post, it would seem I just about prefer Sygic but you pays your money (I hope) and makes your choice.....