Phone GPS or stand alone

I want to get GPS for when I drive and I was thinking about a Garmin Nuvi, but I am also thinking about getting an iPhone, which I understand has a good GPS feature. It might be a hassle to use the iPhone in the car, but it would save me the cost of the Garmin. Is the iPhone GPS any good as a mobile GPS, or does the Garmin have a lot more features and would be better for what I need it for?