Choose the image sensor before judging a camera by its megapixel label. ITU-R BT.2020 specifies UHD at 3,840 × 2,160 pixels, roughly 8.3 million; 1,920 × 1,080 contains about 2.1 million. That is four times the pixel count, not four times the useful detail. Sensor area, pixel size, and exposure also shape night footage. Pixels aren't everything.
The EMVA 1288 camera-characterization standard measures sensitivity, temporal dark noise, and dynamic range. Use those measures to compare sensor performance, not marketing adjectives alone. A wide lens captures more roadside context, but distant plates occupy fewer pixels. It can also bend straight lines near the frame edges. Check the stated angle of view and inspect sample footage, especially at night.
Video quality depends on the whole chain. A sharp lens cannot rescue noisy sensor output, and heavy compression can erase fine detail. Compare clips of the same moving scene, viewed on a large screen: look for readable signs, controlled headlight glare, and clear edges. Test at night. I would not choose from daytime samples alone; tunnel exits and wet roads expose weaknesses that a bright showroom hides.