an easy answer Z.
A better answer, depends on the board, water/spot condition, rider weigh, riding style, sail trim and what purpose you sail (figure 8, downwind, speed). The Sting 2 is an overall good fin, but could slip sometimes upwind, the Z-fin is stiffer and needs a bit more attention downwind but have a better "bite" up wind. But it all comes down to who, where and what.
www.andersbq.com/2013/03/obsessed-with-fins.html