The difference will be during the first and last minutes of legal hunting time, when your eye increases its pupil to the largest size. The 50 mm objective will allow you to use a higher magnification, without loosing brightness. The specific benefit of 50 over 42 mm depends somewhat on your age, because the eye's pupil size decreases with age.
For example a person in his 30s probably has a 7 mm pupil in low light. A 50 mm scope can be turned up to ~7X mag and be as bright as the naked eye. Above that mag the image looses brightness fast. A 42 mm scope can only be turned up to 6X mag without loosing brightness.
If both scopes are set to 6X the brightness will be the same, but the 50 mm scope will have a larger exit pupil and therefore will be easier to align to your eye in low light.
This of course assumes all other factors are the same, such as lens transmission (similar anti-reflection coatings). Tube size is not a factor in scope brightness.
Last edited by bruce_ventura; 08-02-2013 at 09:15 AM.