The benefits of portable GPS Navigation systems (Sat Navs) are some what's obvious. Portable systems are easily transferable from one car to another, and due to advances in batteries, can be transported and used outside of the car. So if you have two cars in your possession, or hire cars on a regular basis then the portable Sat Nav system is one you really need. In addition, most modern systems plug directly into you home computer, usually via USB. So maps and system software are more easily updated.
With this in mind, the in-car Sat Nav systems seem to have little advantages compared to the portable systems. However, for the single car user, the in-car Sat Nav removes the need of plugging in a Sat Nav unit for every journey, as leaving a portable system in a car is a security risk.
Fixed in-car Sat Navs also have larger and higher quality displays, usually as they do not need to be attached via a suction cup to the car windscreen, like their portable counterparts, which will only only hold up to a certain weight. So less cables and wiring cluttering up your dashboard. Fixed (in-car) systems which use CD or DVD to deliver their mapping data, also double up a DVD player, and some are supplied with a remote for control via back seat passengers. Manufacturer installed Sat Navs are even more integrated into the vehicle, and can also operate the radio and climate control.
In the past features such as bluetooth, POI (points of interest) and touch screens used to be in the real of in-car Sat Navs only, but are now available on mid to high-end portable GPS systems as well.
A major benefit, which is often overlooked, as it is not obvious just by viewing the two systems side to side, is that the fixed systems are a lot more accurate.
Before, explaining this advantage, a little knowledge is needed on how GPS systems know where you are. GPS systems, use receive signals from three separate satellites, high up in orbit around the earth. Three satellites are the minimum required mathematically due to a process of triangulation that will calculate your exact position given the relative distance from the GPS unit to the satellites.
Both the fixed in-car and portable systems use satellite triangulation. However, in-car Sat Navs, also gather data from the vehicles electronics known as speed pulse or Vss. The speed pulse signal is typical used by the cars dashboard to display the speed of the car. In addition, there is a gyroscope embedded in the Sat Nav, that provides information on what direction the vehicle is turning. With this additional data, the GPS knows what speed and direction you are traveling much more accurately than using satellites only. This is especially usefully when the satellite signal is weak, or you are traveling through a covered area such as a tunnel. The problem is that with the extra cabling requirements, fixed systems need to be installed by an experienced mechanic.
In conclusion, the portable Sat Nav units are more convenient and accessible, where as the in-car Sat Navs are more accurate with larger screens, but require a more complex installation procedure.