I use both; Google on my phone and HERE on my car's built-in nav unit. Being able to use the car's system where there's no or poor cell coverage is definitely a big plus. However, the HERE data is old and incomplete as far as businesses. If you know the street address of where you want to go, it's great. However, if you just want to look up all the Walmarts nearby and pick one to go to, it tries to direct me to one that's an hour away for some odd reason. And forget about finding some small restaurant; if it's been there for decades, it's probably on HERE, but if it's fairly new, forget it.
Google Maps' biggest strength is that it combines navigation with an up-to-date business directory. I can search for "Italian restaurant" within a certain area, see all the businesses that match that description, then look at them and immediately see peoples' Google and Yelp reviews, so I can avoid places that suck. Then I can just tap one button and have it navigate me to that place, without having to mess around with street addresses.
I wish my car's system could integrate the business-directory stuff from my phone (assuming I have coverage at that moment), and then switch me over to the car's navigation after it gets a street address.