Although I think you will see a slight improvement in jumping from 42mm to 50mm if you are comparing the same exact model, I also think each model with its specific coatings is going to have just as much effect on the difference as the extra 8mm in the objective lens.
I recently compared the following in low light conditions off and on over the last several weeks (30 min before sunrise, overcast day at sunset, 30 min after sunset, half moon, 3/4 moon, no moon) on objects from 30-600 yards. I don't currently have any 50mm so it was all comparing 30s, 40, and a 56.
Zeiss SF 8x42
Zeiss SF 10x32
Euro/Meostar HD 8x32
SLC 8x56
Swaro EL 8x32
Swaro Habicht GA 8x30
Swaro Habicht GA 7x42
Swaro Habicht GA 10x40
NL Pure 10x42
The brightest was the SLC 8x56 which was no surprise. The next in brightness and seeing detail was a tie between the Habicht 7x42 and Zeiss SF 8x42. The 7x42 might have been a little brighter but the 8x42 had a noticeably wider FOV as well as the 1x increase in magnification which made up for any slight advantage the 7x42. The Habicht and Zeiss SF are two very different animals.
In the 30mm group, the Habicht was just as bright and crisp as the EL (in the middle of the image) and noticeably brighter than the Euro/Meostar HD. All 3 were brighter than the Zeiss SF 10x32 due to the 8x vs 10x magnification.
The Habicht 10x40 was much brighter than the SF 10x32 and was just as crisp as the NL 10x42 in the middle of the image. The NL's huge FOV made it look brighter. The strange thing was that even though they are both 10X, the NL's magnification looked a little larger. This might be due to the difference in porros vs roofs.
Also take into consideration not only when you will be using them but also how you will hold them. I can handhold the SF 8x42 more still than the big SLC 8x56. So I end up seeing more detail while handholding. If I was just hunting for a blind and had the optic on a tripod, I would go for the big 56mm and not worry about 50mm.
