When you use machines you know that you have the correct posture and are protected to some degree from injury, and the machine will take some of the weight that is being lifted. If you use free weights you have to make sure that your posture is correct at all time and that your body is not lifting weights that could cause injury by incorrect usage or lifting to heavy. I prefer free weights, I feel that my workout is more beneficial,its easier to do supersets and I am not wasting time waiting for a machine to become free