Which one looks better at night? Ride around the same area, same speeds, same traffic level with each option and see what gets you the most useful details. Also consider how long it takes to transfer to your computer, how hard it is to process each file size, (sometimes you can squeeze out a bit more detail with different types of postprocessing, just make sure you preserve the original in case it's needed in court) and any other factors.

Short of having some pro quality gear, it's going to be hard to get plate numbers in anything but full daylight anyway. Especially in the case of rear-facing cameras where all they're going to see is headlight glare.