imo, the front/rear debate can be summed up simply here...
front shackles perform better as when the spring compresses (hitting a rock) the axle moves forward slightly, helping the tire bump over the obstacle, where as with a rear shackle, it moves away from the obstacle.
REAR shackle is better for beating, basically if your shackle hits an obstacle (on the front) you're going to bend the spring in a way it isn't designed to bend, and most likely break the spring... this can be counter acted by a skid plate that covers the shackle completely