Here is a helpful forum on dash cams.
https://dashcamtalk.com/forum/
I did a lot of research here - and there is a nice section called "what should I buy?". And there is the "Best bang for your Buck" comparison guide.
I got the Street Guardian for my car. One of selling points was for me, is the operating temps the unit is designed for. I'm in FL so the inside of the car can get pretty hot when parked in the sun all day. The Street Guardian has very high heat tolerance.
Other things to consider, the clarity of the video (day and night view(, ability to auto-record upon movement (i.e. vehicle is parked and someone hits it while your not there), the ability to add a second channel (camera) for rear recording or interior. Company support (firmware upgrades, help, etc.).