I really think it should be dependant of the car make, and the age. Hopefully GTA V will have more reality.
For example, BMW's and Mercs that have the fin style antenna:
http://i52.tinypic.com/x2w7c1.jpgOlder cars with an extending/stationary antenna...
and old busted up cars in poor condition with a coat hanger antenna...
With police cars I'd like it if they had mutliple antennas, etc.