Google Maps is good for pre planning in conjunction with Google Earth. But in car use it is expensive as a Internet data user ( for most of us outside the USA) . Should you run into bad weather signal reception is weaker then GPS.
Hence Stand alone Garmin or I now use Sygic on my Galaxy 7" Tab with onboard (TomTom) maps, small cost to purchase by Region.
I have used Sygic in all weather conditions, great product and you don't have to buy a stand alone GPS Unit.