Handheld GPS vs Portable Car GPS, which is more accurate?

I'm planning to buy a new GPS for backpacking travel, I was wondering compare to Portable Car GPS, is hand-held GPS more accurate?

Car based systems won't be any good as soon as you leave the highway. Handheld one's don't mind where you go. And free openstreetmap mapping means you can often download up to date maps for nothing onto handheld devices. And use your GPS to improve on this open source mapping.

OBD GPS tracking device helps in tracking real-time location of the car using global position system. A SIM-based device that is used to send data to the server via GSM network! The user can remotely monitor the location of the car on web application or mobile app. GPS location helps owners of large fleet or personal owners to track the location of a vehicle. GPS location helps in tracking your vehicle anywhere & anytime.
visit: http://www.thinkrace.com/obd-tracker/