As onetrack says,each person will have different priorities when selecting a dashcam.
I chose mine based on the tradeoff between image quality and physical size.The image quality has to be good enough for you to clearly read number plates on the video files.No point buying a low resolution unit.I looked at online videos recorded with the unit before buying.Mine is also the smallest HD unit on the market and I can swap it between vehicles using a small strip of velcro stuck on each vehicle's windscreen instead of a large bracket with a sucker pad. (Size: 63 x 43 x 21mm and weight 66.3g).
I wouldn't recommend it though,the price is ridiculous (£199.00),the audio mute function doesn't work and support is rubbish,but it does the job for me.