The aperture is how big the shutter opens up. It is a photography term. With a nice SLR camera, like NIkon, a large F-number (like say 22) is a very small hole (why large number is small hole is...who knows, maybe just to confuse people) and lets in a small amount of light. But it has a very deep depth of field (things are in focus over larger distances). A smaller F-number (like say 1.2) is a bigger hole and lets in more a larger amount of light. But it has a very shallow depth of field (things are in focus over a very short distance range).
I doubt you could tell the difference between these two cams in the daylight, even if they were side-by-side. But if you want color in low light, the Dahua 5442 should perform better.