If you are talking modern sabots then a 20 ga is fine. I just sold my slug gun (12 ga HR Ultra) and if I were to get another dedicated slug gun I would get a 20 ga. With the modern sabots the ballistics are very similar to a modern inline muzzle loader. More than enough to kill deer beyond 100 yards. Now if you were talking smooth bore I might suggest the 12 over the 20 because the rifled slugs have pretty poor ballistics and the 12 ga would actually offer an edge. However with modern sabot slugs I don't think the 12 ga offers you anything but more recoil.

I also might suggest a 12 if you were going to use the gun for multi purpose hunting, like if you were getting a combo model with a deer barrel and smooth bore barrel. However in reality a 20 ga smooth bore will do everything a 12 ga will as far as velocity goes, it just throws a few less bb's to do it.

The BPS's are nice slug guns I think you will be happy either way. My next pump WILL be a BPS.

Paul