The low-light ability is mainly based on dividing the objective diameter by the magnification. That give you an exit diameter for the image. (Quality of glass also factors in, but it would be close to the same from the same line of scopes)
So, for low light, you want to crank the power down as far as possible and get the biggest objective you can.
At 16X and a 50mm objective, your exit image would only be 50/16 or 3.125 mm.
Your pupil (depending on your age, eye condition and how much light exposure it has been recently exposed to) will range from 2 to 9mm.
Refer to this:
http://www.sizes.com/people/pupil_size.htm
So, at 16X a typical 70 year old wouldn't gain anything from going to a bigger objective, but if you're 20, you would want to reduce the power to 6X and get the 8.33mm image, or increse the objective lens to 128 mm (that's not an option for a rifle scope, just an example). /ubbthreads/images/graemlins/smile.gif
It's all trade-offs. I look at it this way: if it's that dark, I'm not going to spot something 500 yards away to begin with, so I'm not going to need 16X to hit what I can see. I'm satisfied with a 50mm objective.