Both are early read balls.  Flux Solid reads a touch earlier for me with a softer move down lane.  Volatility is still an earlier read but I can get much more shape down lane.  For me the Flux Solid is better playing tighter angles through the fronts or on shorter patterns where you are looking to have the more control after the break point.  Volatility is better when giving up the pocket and shaping the ball (this fit my eye better), I see more play with it then a Flux Solid.