0.20g BBs are too light to be used outdoors, they have a habit of bouncing off leaves and dead grass at the worst of times, so imagine what a 0.12g would be like! The brush at the fieild where BW is played has really thick brush, and long straight trails. You won't get the range nor the penetration with either of your ammo choices. Use minimum 0.25g.
And consider this, it might take you a hundred rounds of .12g to get a kill, or 25 rounds of 0.20g............. or a quick burst of 3-5 rounds 0.25g BBs if the target is behind foliage or past 100ft. Don't go cheap on getting your kills.
I don't even use 0.25g (except for indoor games) ourside, I use 0.28g in my AEGs/pistols, and 0.30g to 0.36g in my sniper rifles.