Lighter ball, up to a point.  The third variable is the amount of rotational energy you can get on a ball.  At 12 mph you cant preserve rotational energy no matter how many revs you have in a heavier ball.  Going lighter not only gives you the opportunity to up your ball speed ( a must if you want to improve carry ) buy allows you to get more revs.  When I say lighter to a point, below 14 lbs starts to have other trade offs.  Also if you don't have an effective release, going lighter is not going to help you get more good revs.