I would first ask where you live. If in a rural area with no neighbors across the street, I'd choose slugs. If in an urban envrion, 00 buck. If you choose shot, keep in mind that you may need the penetration offered by buckshot. While any birdshot work well up close on an unprotected body, if the BG is behind light cover, wearing heavy clothing such as a leather jacket, etc., birdshot will probably not do the trick and reach his vitals.
Back in the day, I patterned six Remington 870s for my department. Despite having consecutive serial numbers none patterned exactly the same. Choose what type of ammuntion suits your purpose, and then buy several different brands and pattern them in your shotgun. Purchase more of whatever works best.