If You want light gathering and that is your main goal, then a fixed power scope of high quality is the thing. A 4x40 or 6x56 with a 30mm tube made by one of the European m,anufacturers like Zeiss, Schmidtand Bender, Swarovski, and IOR. Expect to spend 400-1000 bucks.
You want to pay attention to the size of ther exit pupil which is the objective size devided by the occular size IE a 10x50 binocular has an exit pupil of 5mm (50 devided by 10 = 5. The bigger the better.
And the higher quality thescope, the better the glass and coatings. Hard to beat Zeiss' and Swarovski's premier coatings although I hear that Nikons premier scopes are azbout as good.
My hunting partner who is a bit old fogeyish says that you should spend 3X as much on your scope as you do on the rifle.