Re: Why a 50mm objective ?
Well, I would have said there's far more to it than simply comparing exit pupil diameters.
Comparing 40 to 50mm is only relevant if you are comparing scopes of the same quality.....one of 'China's Finest' scopes in 50mm will not be as good as a quality 40mm.
(Once you are comparing scopes of similar quality) do some reading (web search) on 'twilight factor' and resolving power.
.....and remember that the surface area of the front lens goes up by the square of the radius so (as with Chris's point) small increases in diameter give BIG increases in surface area....hence the often seen military compromise of 42mm. Talking of military, the Brit Army switched to 50mm scopes some time ago; I believe the USMC have done the same.
All that said, it's only a real issue if you anticipate a requirement for decent magnification when shooting in poor light (in which I would include heavy mountain overcast...not just twilight). If weight's more important to you, you should save a few ounces with the narrower objective.
...as regards 'AO', revise the question to 'parallax adjustable' and I would say definitely get it; it makes your scope forgiving of inconsistent head position/ eye alignment. An actual 'Adjustable Objective' is a non-starter for field use (IMHO) because you will have to break position to reach forward to adjust it, side turret parallax adjustment is the only way to go.