Generally speaking, an increase in MP causes images from low-light conditions (i.e. night) to decrease. It's harder for the camera to capture light as the number of pixels increase. So in bright situations, no problem... but as the light decreases, the image on higher MP cameras usually starts to get grainer and grainer, with motion often becoming an unusual blur.
The best cameras for low-light conditions are Dahua's Starlight and Hikvison's Ultra-Low Light series. These are typically 2MP cameras that utilize a Sony STARVIS image sensor.
If you haven't played around with it already,
HugeDomains.com - Iptvm.com is for sale (Iptvm) will let you find your property on Google Maps, drop a camera on the map, then shows you what the daytime/nightime view would be like.
For doing faces/license plates from the roof, you're probably going to need a camera that can zoom in on the sidewalk/street. Look in the Dahua forum here for threads about LPR (license plate readers). There's a Dahua Starlight bullet that I think ends with Z12 that seems popular for this task.