This is an update (correction) to earlier post: Re: Auto GPS Suggestion and attached discussion:
One good feature of portable GPS is that you can obtain updates and different maps via your computer via the company from whom you bought the device. If you purchase a new auto with GPS built into the dashboard, the dealer has to provide updates. * Also, should you lose your GPS while traveling internationally, it is not as costly as if you were to lose one supplied by the auto rental company!
One correction regarding downloads: At least with my TomTom (as shown) you don't have to download the map loaded on your GPS to your computer; you just connect to the company (TomTom) and delete the installed map and purchase the one you desire (i.e. for "Western Europe"), and it will download onto your GPS (takes some time to do so), and your former map will be stored in your file at the GPS company (such as TomTom). Later, when you want to reload your former map (such as US and Canada), you repeat the process. Once you purchase these respective maps, or additional features, they belong to you.
Many people use their Iphone maps/GPS, but I prefer the larger model GPS units with audio, night screens, directions to gas stations, etc.
Hope this is helpful.