I actually like some things about both Navigation Apps. Sprint Nav is more pleasing to listen to than Google Nav's robotic voice. Both seem to give similar accuracy.
The one thing I like about Google Nav that Sprint Nav doesn't do is, it shows a street view of the location you're trying to get to (at the end of the route), so you know exactly what you should be looking for.
I have no real complaints about either. I've experienced the similar amount of lag for both Nav systems when I'm supposed to be transitioning to another freeway or making a turn soon. Sprint Nav maybe slightly slower than Google, but either has been enough to cause me to miss a turn. Overall, both are very good to the point that I never felt the need to get a dedicated NAV system.