Re: 40mm vs. 50mm
I'm not sure that anyone actually answered your question.
One consideration in evaluating the size of the objective is how you will shooting your rifle. As you have indicated, if you decide on a large objective, you will of necessity have to use higher rings. If your shooting consists of shooting off a bench, this should present little difficulty. But if you intend to shoot prone, you may find that high rings will not allow a proper cheek weld. I know that for me, my comb-eye distance is subtantially reduced in prone requiring me to elevate the comb of the rifle to obtain a proper eye/scope relationship. So, from the bench high rings may work fine for me; but when I try prone, I have to raise my head to the point that sometimes I will not have any head contact with the rifle at all. Not good. Therefore, I have found that in order for me to shoot prone, it is necessary that I use short rings - and that means that a 50mm objective is out of the question.
To accommodate my particular needs, I have settled on a USO scope with a 44mm objective. The glass is of good quality, coatings are first rate and enough light is admitted to shot well after sunset. Therefore, the 44mm objective is both practical and adequate for my use.
You may have other demands that require a larger objective. If so, your scope will necessarily be a little higher. Make sure that this added elevation is compatible with the positions in which you intend to use your rifle.