Definitely go with the Garmin over using the cell phone for GPS. You will have a hard enough time keeping her off of the phone while driving anyway. Plus there is an iPhone app I believe that won't allow her to use the phone for texting/ talking while the gps in the phone senses movement over a set mph. I don't have an iPhone (I'm an Android guy

Also when the phone is used as a GPS it eats batteries. Now if there is an emergency/accident the phone has a greater chance of being dead. Good luck with your decision, I've yet to see/use a Garmin GPS that I didn't like as far as ease of use and features.
More importantly, good luck with the 16 year old daughter...