I prefer using Google Maps because it (currently) has a better visual look especially the depiction of traffic. It's very easy to tell where the traffic is heavy and where there are incidents. Since Google bought Waze, the traffic in Google Maps is updated quickly and is very accurate. I also have tons of locations save in Google Maps which are visually "starred" on the map which makes them easy to locate.
However, since my new truck has CarPlay, I have been getting used to using Apple Maps, and I have to say that it has performed quite well. The traffic information is there as well (not sure what traffic source is used) and I have even had it (accurately) re-route me around traffic to save time. I'm not particularly fond of the way Apple Maps has your "Favorite" locations saved. And the fact that they are at the bottom of a long list of places for which you have searched seems backwards to me. (Yes, I know that the searched location can be deleted and I do that, but to me, the Favorites should be at the top.) The visual experience of Apple Maps could be improved a bit (strange for an Apple app to have to say that) but overall, I have actually had a good experience with it.
If Google Maps were to come to CarPlay, I would probably use it because of the number of locations and such that I have already saved in it. It remains to be seen whether or not it would perform as well (or better) than Apple Maps in that regard.