The amount of light that comes through an optic is related to objective size AND magnification. You have to also consider how much light the human eye can use. A 20mm front objective set on 4X will let through exactly the same amount of light as a 40mm objective set on 8X, or a 50mm objective set on 10X. And all of those will let about all the light through that MOST human eyes can use. Some people with better than average vision can use more light. If you're over about 30 chances are a bigger objective than these won't help.
Taking the front objective size in millimeters and dividing by the magnification gives the diameter of the beam of light exiting the scope in millimeters. All of the above examples comes in at 5mm, which is as much as most of us can utilize. Some people can use 6 or 7mm, but this isn't common, especially as we age. Going to a larger objective allows us to take advantage of more magnification in low light. I've had no trouble seeing well enough to shoot 30 minutes before sunrise with a 20mm objective on 4X out to 100 yards. If the range were greater and more magnification were needed then I'd need a bigger objective.
The 1.5-6X42 will give you 7 mm of light on 6X and should be plenty. The 1.7-10X42 would be borderline on 10X, in low light, but as long as you don't go over 7X or 8X in low light should be fine too. Having the ability to go up to 10X in good light might make it better for the longer shots.
