The standard (Forster? Foster?) type slug flies (relatively) straight, because it is nose heavy, like a dart. And so does the Brenneke (sabot) type slug.
In a rifled barrel, the sabot of the Brenneke slug provides a large bearing surface for the rifling and the slug is spun, enhancing the accuracy.
The same rifling will (or should) spin the regular slug, but does not usually do so as well as it does for the sabot slug, because of less bearing surface in contact with the rifling. Some slugs might not, or just barely get spun, depending on tolerances, due to less contact with the rifling.
Classic wisdom is that slugs (the regular kind) perform best fired from cylinder choke bores. And while not as good for accuracy, can be fired from any choke barrel. I don't know if that is so, but I can tell you that a slug barrel (smoothbore, cylinder bore) shoots better groups with slugs than a regular choke barrel.
Personally I think this is mostly due to the fact that slug barrels have sights, rather than just a bead, which means the shooter can be more consistent in their aim, so better groups.
Rifled slug barrels add another level of accuracy potential, and while not as good as an actual rifle for groups, they shoot better than smoothbores, with slugs. The down side is a rifled barrel does very poorly with shot. SO a rifled slug gun is slug only, (for best performance). If you have fairly modern gun that allows easy change of barrels, one for shot and another for slugs is a very good choice.