For me it's the traffic display. iOS maps only show bits or small portions of traffic flow. Googles maps show the traffic pattern for every single freeway (in the Bay Area) and that's critical for me. From work I can take 3 different ways home,so depending on the traffic my route will vary. Since iOS maps don't display all the traffic info I need,I'll never use them again. That and in my personal experience iOS maps hasn't been very accurate.
As far as navigation apps (I bought navigon) I'm done with those too. What I don't like with those is you need to know the address to where you want to go.
Now if I'm just looking for the closest Starbucks I can type in Starbucks in google and iOS maps and it will show everyone around me by proximity. That's something you can't do with navigation apps that I've seen. Or you may be able to search for a restaurant,but by the time you've done all that,googles maps will have done it in half the time.