Apple maps give better directions whilst driving and the overall UI is clearer with better contrast of street names and buildings. The directions in front of the green / blue background are great because they look just like real direction signs it makes it easier when driving.
But Google maps has better information, places, POI, reviews. Just not as good UI when in a car and the spoken directions can be a bit late -- for example not telling me what exit to take on a roundabout until I've entered the roundabout.
Both are missing stuff, like speed cameras locations, speed on a lane and spoken directions could be improved - for example it could say take a left at the roundabout, and then stay on the right lane for a right turn in x miles.
Apple has better UI and spoken directions; Google has better information.