You need one with both forward and rear facing cameras.
A large proportion of prangs are rear end shunts.
You need to be able to identify the culprit.
Also if you hard wire them like I have, every time the ignition is turned on it records whats going on outside
or who ever is behind the wheel at the time.
I sat through 16 hrs of playback in January as I needed to find something.
Mine had recorded everything from August onwards.
I found what I was looking for and a whole lot more!