I have a tomtom in my Jeep and an iPhone on the seat. Here is my argument for a dedicated gps -
First, I don't want to use a dash mounted phone. It's a personal choice, sure, but if I get a text, call, email, alert or whatever, then you're leaving the gps app to look at that text, take the call or whatever. I'm not a fan of that. I'm sure some people will say use a hands free device and I shouldn't be reading texts in the car. I have a BT headset but it's not always charged and when I do read something in my car I'm smart about it. If someone has an issue with that then it's a discussion for another time.
Second, if you don't have the iPhone plugged in, you're going to kill your battery using it as a gps. Occasional use is probably ok but if you're using it every day then you must invest in a car charger, and probably a cradle for it. Decent gps units can be found for $99 with bigger screens than the iPhone. If you're buying a charger and dashboard mount, you're 1/2 way there.
Data or storage. Depending on the gps software you're using, you're either downloading maps all the time and/or they're taking up storage on your iPhone. If space is an issue, a gps app will make it more of an issue.
I'm not saying everyone should get a dedicated gps. It's right for me and I thought I'd throw out a couple of things for you to consider.
Congrats on the car.