One is no better than the other. Like most things found in training, the purpose will help inform the best tool for the job. Dumbbells and cables have different ways of applying resistance to our bodies. Dumbbells utilize gravity (a natural force driving things down toward the ground). Cables provide the evenest form of resistive tension, applying the resistance directly in line with the cable itself. To maximize your training, you should utilize a multitude of training methods which will include free weights, cables, and machines.