When I shotgun hunted I used an 11-87 with a Leupold 2x7 shotgun scope, mounted on a cantelever rifled barrel. Definiately go with an auto if you can as it makes sighting in much more enjoyable by soaking up a good bit of recoil. A pump gun with slugs will kick the snot out of you at the rifle range! I picked up 6 different boxes of sabot ammo and went to the range to see what shot best, ended up with Federals and can put 5 rds in about a 3 inch circle at 100 yards. If the situation was perfect I would go out to 150 yds with this combo but not much farther due to slug ballistics. A 12ga slug is a very deadly round within reasonable range, but drops like a brick past 125yds or so.
With a smoothbore and Foster style slugs I would keep it to under 100 yds as a general rule.