The choice here is whether you should choose a clearer resolution or a slightly less clearer resolution.
6MP = 3072 x 2048
8MP = 3840 x 2160
The higher the resolution, the clearer the image and thus potentially more useful to identify unsavoury activities.
With better compression technology these days, the impact on storage of the clearer 4K cameras should have very little impact on the recorded footage.
Here is a video which compares the resolutions on a number plate. Although 6MP doesnt exist as a test, just imagine it as being halfway between a 4MP and an 8MP clarity.
It is important to note that MP (megapixel) refers to the resolution of the camera’s sensor, and higher MP count generally means that the camera is capable of capturing more detail and producing higher quality images. In this sense, an 8MP camera would generally be superior to a 6MP camera in terms of image quality.
Other things that are important to consider these days is if to consider a colour at night camera, or black and white at night as these will have a big impact on the quality of the image at night.