If your car doesn't have built in GPS - do you prefer to use a stand-alone device or an app on your phone?

We're shopping for a new car and can't get the built-in GPS without some other serious upgrades - just doesn't make sense financially. My old car has built-in GPS and I'm embarrassingly dependent on it. I'm not sure whether to buy a Garmin or just something to mount my iphone on the dash. I'm a little worried about the phone with the smaller screen / more distractions.