First check your regulations, what features are banned, what is required by the regulations, there are big differences in different places.
Think about voice recording, you may want a good microphone for recording the rear, but in some places audio recording may not be allowed.
How many video channels do you want, are you recording the interior, or is that not allowed?
How long do you want to keep the recordings? Only when you press the button, 1 full day of driving, 1 full week of driving in case someone complains about an incident after a few days? In some countries it may be illegal to keep anything that is not required for more than a few minutes.
The answer will be different for every driver and for every city!