Re: fixed or variable scope for low light performance
If you're looking for a "bright view" and most of your shooting is done at relatively close range, a fixed scope can do quite well. They're also usually lighter than their variable counterparts and you can usually get better glass for your money.
However, if being able to see fine detail in low light situations is important to you (spotting hits on targets, facial recognition, counting the points on a buck's rack, etc) variables have a huge advantage, especially if the shooting is more than 50 yds or so away.
The variable allows you to crank up the magnification to the maximum the light conditions will allow. At different distances and in different light conditions you will be able to see the most detail of the target on different powers.
The old "7mm exit pupil" rule of thumb isn't where you usually see the most detail--sometimes it is, but often it's not. As you add more magnification, yes, the view gets darker, but as long as you can still see the target resolution will keep increasing to a point. At some point the view seems to just "go dark" and you need to turn the scope back down again, but usually you can see the best detail right before this happens which is often a much higher power than you would have thought.
For example, using my March 3-24X42 (which is a pretty mediocre low light scope) to count the tines on whitetail's racks out in the open in the 300-400 yd range the last couple minutes of shooting light (1/2 hour after sunset) I find I end up in the 12X-18X range depending upon lighting conditions right at the end of legal light.
A scope with a larger objective (assuming also quality glass) in any given condition is going to let you turn the power up a little higher before it "goes dark" allowing you to see more detail. This is where the 50mm, 56mm objectives really shine. On the lower powers they're pretty much wasted but with a variable you can crank up the power and make the most of them.
It's also the reason tests by gunwriters where they set all the scopes at 6X (or whatever) and look through them in low light are meaningless to the scopes' abilities when actually used in the real world.