If you are shooting saboted slugs out of a smooth bore accuracy is going to suck regardless of choke. The saboted slugs are made for a rifled barrel. You would be better off buying several brands of rifled slug and see which are the most accurate out of your shotgun. My 870 like the Federal Tru-ball slugs.
As for choke, a rifled slug can be shot out of any choke, but IC, Skeet, or cyl. Bore will give you the best accuracy.