There are gobs of sat nav models to choose from. While some stick to their job of getting users to destination, others are actually in car entertainment and communication systems with competitive price tags.
 
Portable or fixed?
The fixed sat nav models is installed to the car by car manufacturer itself and is substantially more expensive than portable models. The fixed sat nav has got an advantage of being less dangerous for theft and loss. The portable sat navs are more flexible sticking to users where ever they go and can be used when hiring cars abroad and take it when on a walk.

Beware of heat reflecting windscreens
Some newer models, especially French manufacturers fit sat nav with heat-reflecting windscreens. It is been revealed that these radiation are actually hindering the capability of the sat nav to receive GPS signals. As a precautionary measure, test the sat nav before you buy.

Map coverage
The map coverage that comes within your unit must be sufficient enough for your requirement. Always check for an easily extended coverage model for future convenience.

Points of Interest
Check out the set up POIs in your package and also whether you can add and set POI particularly tailoring your need. 

Safety camera alerts
If you drive over speed, it is high time to be overcautious due to the increasing number of speed camera that lie along side the streets. Better check for safety camera alert in your sat nav to go by law and avoid unnecessary penalties.

Live Traffic Updates
Frequent road users who get disappointed with roadworks, accidents and traffic will find this option more useful.  They give live updates and suggest users with the best alternative route.