Machines are usually less effective than free weights because free weight require you to balance more and this factor makes the preacher curls with dumbbells more complete. however, there’s one disadvantage all free weight and cable biceps curl variations have that a machine doesn’t – gravity!
During any style of biceps curl, the point in which the muscle is maximally stimulated, is when the lever arm is at its longest. This occurs when the forearm is at a 90-degree angle with the load vector.
On the other hand, the biceps curl machines is designed with a CAM system, the CAM is set up to offer you a much more consistent resistance throughout the entire range of motion.
The preacher curl machine is an exercise that hits the muscle from all points of the elbow flexion range.
Another benifit, is if you are a begginer and don't have a proper form yet, a Machine will be better and give you less chances to cheat.