What are the advantages to a 50mm over a 40mm? Thanks

Given equal quality of glass and same magnification the 50mm allows a larger circle of light transmission to the pupil of the eye. Known as "exit pupil".

Exit pupil in mm = diameter of the objective lens divided by the magnification
With a 9x50 scope the exit pupil is 50/9 = 5.6 mm (more light),
compared to a 9x40, 40/9 = 4.4 mm (less light).

There is a practical and functional limit to this advantage. When the exit pupil size exceeds the actual diameter of the pupil the eye cannot use the additional area. So somewhere around an exit pupil of 7 you will achieve the maximum amount of light possible.

Don't forget you can go in between both an go with the 44mm objective. What were you wanting to hunt ...ie: quarry/terrain?

Larger bell scopes are more prone to damage and getting knocked off sight..More expensive and only a little better in low light conditions..

none.

50 mm bell is bulky and awkward...Small return for the investment.

Heavier too, so if your going to carry it all day, 40mm is better.
I've got 50's and even a 56mm, but they are not on my deer rifle.

Thanks guys, looks like 40 it is!

A 50 usually requires higher mounts also. I prefer to use lower mounts and have the scope closer to the barrel than higher. Keeps things a little more compact

Like most fishing lures..... made to catch customers not fish. 50mm imo causes more problems(a few stated above) than worth.