I used both when planning a trip to NYC last summer.
google maps was great for before we left to kind of give us an idea of where we were going.. but I used my iphone map while on the road... The iphone map was good while we were actually in the city. It was very helpful while trying to find specific museums, resaurants, etc... but it was very slow to load when we were actually on the road to our destination.. Be careful about your date usage as well..I thought iphone maps wouldn't use as much of my data. I even added a roaming package as I was in the US and I'm from Canada, but still came home to a $400 cell phone bill.