Google Maps is far easier to use, has more accurate GPS, does not require the user to pay for a patch to get directions, and is faster.
You obviously haven't used the latest OVI Maps on one of these devices have you.
The interface for OVI is much improved over what it was. Turn by turn navigation is free on OVI maps, you can't even get turn by turn navigation at all on Google maps unless you are in the USA, and then only on maybe two models of device. Over 70 countries, 11 devices and counting for OVI.
The GPS fix will only ever be as good as the receiver in the phone, however if you mean the accuracy of the maps then on a quick check the Navteq ones used by OVI seem more up to date on a few newer changes I know of, and OVI is certainly a lot faster to display the maps on screen, especially when I am in a mobile network blackspot.
Not that OVI maps are exceptional, some of the routing algorithms could do with tweaking, it likes to go up back roads in town for me, and it seems to think it will take 35 minutes or so to travel the 8 miles on country roads to my in-laws.