More accurate,longer range. Only down side is price per round . So if you don't plan to take longer shots you can save with the smooth bore barrel .
From wiki .
Saboted slugs are lead-cored, full copper-jacketed or solid copper projectiles supported by a plastic sabot, which is designed to engage the rifling in a rifled shotgun barrel and impart a ballistic spin onto the projectile. This differentiates them from traditional slugs, which are not designed to benefit from a rifled barrel (though neither does the other any damage). They can take the usual variety of shapes, but for maximum accuracy are typically bullet-shaped (ogive). The slugs are generally significantly smaller than the bore diameter, increasing the ballistic coefficient, and use the sabot to seal the bore and keep the slug centered in the bore while it rotates with the rifling. Saboted slugs, when fired out of a rifled barrel, are generally far more accurate than non-jacketed slugs out of a smoothbore, with accuracy to 300 metres (330 yd) approaching that of low-velocity rifle calibers. By 1966 (published in 1966 Gun Digest), Ballistic Research Industries was producing a 440-grain (29 g) .50 caliber sabot slug of a new design. The projectile was a wasp waisted hourglass shape made of a hard lead alloy. While many equate sabot to rifled barrel, this slug design was specifically designed for and tested in smoothbores. Testing even included over/unders and side-by-sides. The same slug, made of zinc and about 260 grains was marketed to police for car body penetration. The self-stabilizing shape allowed it to be used with bore cylinder or improved cylinder barrels.[6][7]