Airsoft shotgun help!! 10 POINTS?

If i were you i would just use the most accurate because if you go with .20 then they are slower but weigh more and will have more weight to hit with, if you use .12 they will be faster but close half the weight to pack a punch. Shoot a cardboard box and whichever makes the biggest indent on the next piece will hurt more. Just weigh your options. It probably wont make enough difference to sacrafice accuracy. Hope it helps.