The advantage of the squeezed/tapered bore effect (which is effectively duplicated through the use of APDS ammo in modern cannon) is the increase in muzzle velocity and consequent increased range/penetration.

I'm frankly not sure how a sabot dropping away duplicates the effects of squeezing the caliber of the fired round down by ~30%. I *suspect* it's because the sabot fills out the larger caliber but is substantially lighter, so the acceleration provided by the gasses of the bursting charge is effectively increased in the final fraction of a second of firing, as the sabot falls away just outside the muzzle.

The squeezed/tapered bore effect would seem to be potentially much the more efficient of the two methods, since you've got the full length of the barrel to use for the purpose of increasing the effective gas pressure. However, APDS won out in practical RL use because it allowed you to use existing cannon (and existing much simpler cannon-boring technology) without modification.

In the context of a future setting with advanced manufacturing techniques - such as 3D deposition of artifical diamond rather than old-style bore drilling - I suspect that squeezed/tapered bore cannon might make a comeback.