If I were you I would go for the built in one. Nothing like a 90K+ car with a portable Nav unit stuck to the windshield. Of course everyone has their own opinion but I like the integrated one way better.
The integration is quite good for the Euro market but sloppy for the USA. (No integral Traffic Adviosry System). It can't be blmaed on Porsche,--it's a universal problem in the disjointed UNITED States.
I have the stock NAV unit but for long journeys which require dozens of waypoints to stay on the prefered track and for useful information like the actual speed etc. I always travel with my Garmin unit as well - yes - a radar, GPS and Sport Chono all on my dash! The stock NAV does show passengers a nice colorful map of where you are - other than that it is mostly useless. The Garming Unit allows you to really plan your route and analyze the directions on your laptop using a variety of resources. To try to do the same planning using the stock NAV would not be possible.