When I first used clay pots for brewing tea, I liked the ball filters over the flat hole type. Because the leaves can not clog the holes and interrupt the flow. Soon enough, I found that the flat hole pots make better oolong teas, specially Dan Cong. Reason being that flat hole pots can empty almost all droplets of water when you shake long and hard enough, while ball filter pots can't and will keep on steeping tea continuously at the bottom. This might benefit pu-erh, but not for oolong. Dan Cong can become bitter/harsh if steeped for long, even just a few drops of water can make a difference.