The higher the resolution the more disk space is used. There's a calculator in the
Wiki in the blue bar at the top of the page that will help you figure things out. Not only is resolution important, but frame rate is another one. Again the higher the frame rate the more space is used.
Don't chase megapixels unless you have a really BIG budget. Chase sensor size and bigger is better. To confuse you more sensor sizes are listed in fractions so do the basic math to be sure, 1/2.7 is bigger than 1/2.8 or 1/3. General rule of thumb is that a 4MP camera will easily outperform an 8MP camera when they both have the same sensor size. Reason being that there are twice as many pixels in the 8MP versus the 4MP. This results in only half the available light getting to each pixel in an 8MP that a pixel in the 4MP "sees".
The three basic rules of video surveillance cameras-
Rule #1 - Cameras multiply like rabbits.
Rule #2 - Cameras are more addictive than drugs.
Rule #3 - You never have enough cameras.
Quick guide -
The smaller the lux number the better the low light performance. 0.002 is better than 0.02
The smaller the "F" of the lens the better the low light performance. F1.4 is better than F1.8
The larger the sensor the better the low light performance. 1/1.8" is better (bigger) than 1/2.7"
The higher the megapixels for the same size sensor the worse the low light performance. A 4MP camera with a 1/1.8" sensor will perform better than a 8MP camera with that same 1/1.8" sensor.
720P - 1/3" = .333"
2MP - 1/2.8" = .357" (think a .38 caliber bullet)
4MP - 1/1.8" = .555" (bigger than a .50 caliber bullet or ball)
8MP - 1/1.2" = .833" (bigger than a 20mm chain gun round)
Disclaimer - These are the sensor sizes reported by the manufacturers and may not be the actual physical size of the sensor in the camera.
Focal length, lens size, is another critical element in camera selection. A 2.8mm lens will produce a nice wide view but will be useless for identification at distances greater than abut 10 or 15 feet.