I agree, mostly. To expand on it a little here is some info I posted previously on the topic (but on 50mm vs. 40mm):kudu58 said:I would choose the 3-9x40. At anything above around 5x, it will gather noticeably more light in lowlight conditions or shooting at night.
So, unless the 33 mm obj. has better glass and coatings than the 40 mm obj., I concur with kudu.Glass and coating quality trumps objective size every time. The reason a 50mm obj. "can" be better in low light is that it increases the exit pupil diameter at any given magnification.
For example, the exit pupil of a 40mm obj. set at 9x is 4.4 mm. The exit pupil of a 50 mm obj. set at 9x is 5.6 mm. Most of us have pupils that will dilate to over 5 mm so there will be a slight advantage.
If you move scopes down to 5x, the 50 mm obj. will offer no advantage as the 40 mm scope will deliver an exit pupil of 8 mm, and is about the most any of us can use.
Basically, what it comes down to is this: in scopes of the same quality, a 50 mm obj. will allow you to use a higher magnification in low light.