Even on a "sporting" rifle designed for a scope (e.g. my Weatherby Vanguard Sporter), I like the 40, because I can use low rings.
With the scope tucked nicely close to the receiver, I get a comfortable, instant, solid cheek weld, which is fine because the Weatherby comb recoils away from your face, not towards it.
The 50 gathers more light than the 40. The downsides? It's heavier, it's bulkier, it has to be mounted higher, and it gets hung up on things more easily in the field.
Light gathering means more at higher magnifications. If you don't need to use the maximum magnification of the scope when it's getting dark, 40 vs. 50 is less of an issue. Many big-game hunters don't even like to use more than 6x, especially for offhand shooting, or shooting with a makeshift rest.
Just going to shoot at the range? Hell, get the bigger scope if you want. But in the field, there are other things to think about.
EDIT: What "gathering more light" means is that a scope with a bigger objective lens will take more light from a larger surface area and focus it back to your eye, than one with a smaller objective. I have to say that I, too don't see that much difference. The quality of the scope matters a lot more. Cheap scopes are usually darker, top-quality scopes are brighter. TANSTAAFL.