It doesn't really matter in crucible, everything one shots in the head and two shots in the body. High impact snipers gain the advantage of easier body kills if the enemy is already injured, low impact snipers have the advantage of higher RoF for quick bodyshots or recovering from missing a headshot. If you're new to sniping use a low impact sniper (every sniper in the tower is low impact now). Once you get good at headshots switch to her benevolence for higher impact