Even though they offer great fit, I wouldn't have thumbhole stock on an rifle with manually operated hammer. It proves to be extremely inconvient until you get real familiar with it, and even then takes more time to cock/uncock hammer. I think you'll be happier with pistol grip stock but BE SURE and handle both if possible before you decide. As to 12 gauge or 20 gauge, get the 1 you prefer and choose the phyical properties of the gun itself instead of gauge, as modern slug loads make them pretty much equal in power & accuracy.