I compared two rem 870 express magnums - mine rifled, the other not.
Using plain jane cheapo slugs, the rifled barrel was of course more accurate. Inside of 100 yards it's easy to get good if you practice. Past that you seem to have to take a little more time to learn how she works, as the slug seems to drop a lot. I also have rifle sights on mine, which helps with consistant aiming.
The smoothbore did well, but not as well. Seems the slugs just weren't as accurate through it, especially as ranges opened up. Also, not having rifle sights added to this.
Haven't gone for bear, but took a deer with my rifled 870 last fall. Very clean kill, and deer was on the ground before I could re-establish my sight picture.