Let me suggest that if you are able to identify your specific requirements it will make your search very much easier.
Need front and rear recordings? Do you need GPS? (Needed to determine and record vehicle speed.) Need to archive any particular duration? Do you really need, say, 1080p? Do you need to be able to playback the video(s) while sitting in the car? Is stealth important - either from a anti-theft while parked perspective or from some other need for secrecy?
I actually have two units in my car. One is the Garmin 20 which has GPS and thus records my speed and of course time, etc. It faces forward and is not very well hidden. I have a second unit, like a $40 set that points rearward and is mounted on my rear window - no GPS, as I have that info on the other unit. By plan it is VERY obvious and i believe it has caused more than a few tailgaters to spot it and back off a little.
If I were shopping today, instead of two cameras I'd look at one of the Split Screen units - half screen looking forward, half screen looking aft. Can go to single screen and use as a backup camera.
Many SEMIs now a days have three cameras. One facing forward, and two attached to the side mirrors pointing aft.
some of these units are pretty cheaply made. I had a huge problem with RFI from that cheap camera interfering with my GPS. No problem now that I have the cheapo camera in the back of the van.