The handhelds have more things they can do than the car units like navigation, hiking, geocaching, have topo and street maps loaded on them. But the car ones (Nuvi and Tom Tom, etc) are bad for hiking and geocaching. I have a five and a half year old GPS for geocaching, so I am not up on the newest ones.
Friends of mine, who are the #4 geocachers in the world use their handheld to street navigate to geocaches although they have a GPS built into the dash of their Accura. They use the factory installed one to find restaurants and gas stations.