I tried something new for the summer. I just mainly CA and it's open hill sides and mountains, so almost all S&S hunting. I got a $40 pair of Celestron 7X50s after reading a book by Dwight Schuh, "Hunting Open Country Mule Deer". He talks about exit pupil and light transmission. I've found that I'm good out to about 700 yards 45min - an hour before Sunrise and it actually makes it brighter. Basically if you can barely make out the trees in the distance the hill side will look like grey light through the glass. The clarity at that range isn't great on the cheap ones, but I can still find them in the beds/shadows, and you can definitely tell if it's a buck/doe. Unless you're going out past half a mile I think 7's are great especially for low light situations. Also, I've found deer faster because i have a bigger FOV so I catch more movement in the edges. I will say I also have a set of 15's and a 20-60 spotter to get in tight after the sun is up.
Don't know if that helps, but if you're looking for low light, from my experience exit pupil is king.
Example: 8X40 = 5mm EP; 7X50 = 7.15ish EP; 10X40 =4mm EP
Human pupil at max dialation 6-8mm depending on the person. If you are under your max pupil size you're leaving light on the table.