According to a study done by Onavo, Apple Maps uses less data than Google maps and even appears to be more efficient! They compared data consumed by Apple Maps on iOS 6 to Google Maps on iOS 5. After a number of different scenarios - using standard view versus satellite, searching for different cities and addresses, zooming in and out - on average, Google Maps ate up about 1.3MB of data, while Apple Maps used only 271KB. When in satellite view, it does use more data on each - an average 930KB for Google; 428KB for Apple.
Not only is Apple’s data rate lower, but in some cases (like zooming in and out) the app actually turned out to be 7 times more efficient than Google’s. Whenever you zoom in and out, data is used to refresh the map. Onavo says Apple has the advantage here because they take a different approach, “Apple’s vector graphics approach resizes dynamically, resulting in the drastically reduced data usage we observed, as well as smooth resizing and fast responsiveness.”
Of course this is great news, but until Apple perfects the app, you may have to stick with Google or use the app over WiFi so no data is used!