Points for the phone (by which I mean, Google Maps on iPhone):
- It's always up-to-date and tends to have better directions
- The interface is nicer
- It's "free"
- Linked to Google so it knows my recent map searches and such
Points for the GPS:
- The mount is nice (magnetic attachment, suction cups on window). Never found a good mount for the phone
- A dedicated device means just leaving it there most of the time, not constantly attaching and detaching it as I found myself doing with the phone (because I need my phone for all kinds of stuff).
- Works without internet
- The screen is a bit bigger
- It's always plugged in
The GPS wins for me so far.