In every case that I can recall the main advantage in buying an option from a car dealership is to ensure that the add-on is functionally or stylistically compatible with the automobile. When it comes to a Garmin GPS (which I highly recommend), there is no clear advantage from buying from the dealership. However, if there is a SMART-specific mounting system sold by the dealership for Garmin GPS, I would certainly be interested in purchasing it. Still, I would buy the Garmin GPS elsewhere.
I have used Garmin GPSs for over a decade and have bought their new releases every couple of years, or so. As the new releases emerged in the market, they arrived with better features at a lower cost. My last purchase was about a year and a half ago. It was the Nuvi 680 (paid nearly $800, but now available for less than $400). It still meets my needs and map upgrades can be downloaded throughout the years (at a reasonable cost). Still, I would look at Garmin's latest from their website and consider buying that (as it would likely have the same or more features than older versions at an even lower cost. The pleasant technology trend of getting more for less cost as time goes on applies to Garmin GPSs as well.
Now what I am curious about is, if there is a Garmin GPS mount designed specifically for the Smart fortwo, who has it, how much does it cost, what models of Garmin's GPSs does it support (Garmin mounting systems vary with models), and how can I buy it?