I would have to argue Dach is a very good defensive player. Dach makes plays defensively using his brains reading plays, Cozen works hard and uses every bit of his size and reach to strip guys from the puck. Cozens also makes a lot of bad plays and turns the puck over. Then again, every shift-by-shift of Cozens I have seen he doesn't have the puck much to begin with. However, he is pretty effective at driving the net when he gets the opportunity and has a great shot to go with it.
Better leader? Have you been in the locker room? Lets not even try to sit here and pretend either of us can have an educated debate on who is the better leader.
Looking back at the last 10 years and at picks 3-10, power forward type wingers 6'3" and above have a scary low success rate transitioning into the NHL as top six players. Playmaking centers 6'3" and above have transitioned respectably well.
Personally, I don't want either player. But I would take the chance on Dach depending on what is available.