I agree with Zoomie.
Most of the time groundhogs come out in the afternoon and well before dark. The advantage of the 50mm bell over the 40 would be insignificant.
The advances in last few years in grinding lenses and optical coating make it a crap shoot if bigger is better.
I have both 40 and 50 mm. bells but I do not hunt anymore. The 40 mm are easier to mount without the need of high or extra high rings and makes for a slimmer look. I would go with the 40 mm.