If you are using a smoothbore barrel, you want the most open choke you can get. Cylinder would be best since you don't need or want any compression of the slug at the end of the barrel.
If you can get a rifled barrel, it would be better. You might get lucky and find a slug that works well in a smoothbore barrel. Some companies like Federal and Remington makes slugs that are designed to work in smoothbore barrels. However, if you can get 4" groups at 50 yards with a smoothbore you'd be doing real well. With a rifled barrel, a scope, and the right slugs you can probably get down to 2" groups at 100 yards.