I normally use Google Maps here in Italy, but while visiting California and Utah last month, I found that Apple Maps while playing music would soften the music while the directions were being spoken, and in the English voice I chose for Siri (more pleasant than the traditional English Siri voice I think), whereas Google Maps would pause the music, then start it up again. I preferred the continuity of having the music get softer with Apple.
Yes, having it built in is a plus, and would account for the numbers. For transit, especially in Europe, Google wins, but Apple has progressed a lot. Apple also incorporates Traffic into routing, which is very nice (Google may also), and actually shows "simpler" roads, which could use less data.