Shooting performance.  There is no upside in spotting shots or straight line recoil to a sloped toe.  The reason that all stocks are sloped is because that’s how they were always made- no one a hundred years ago used a bag under the butt.  It is an anachronism with no real thought behind it.
As for adjusting elevation, it is far better to do that with front rest height, and/or rear bag/rest height.  For the rear, a stiff or very full rear bag is not the way for a field bag.  You want room for the bag to compress, and for you to be able to squeeze it to adjust for height.