• Due to the fact that both hands act freely, your body can make slight adjustments when needed to prevent potential injuries
• Synergistic and stabilizer muscles (known as helping muscles) have the opportunity to gain greater strength by aiding the agonist (primary mover in muscle contraction) muscles
• Dumbbell movements closely resemble the every day neurological patterns of daily living
• Dumbbell exercises can mimic any movement in sports opposed to machines which set the user in a predetermined pattern
• More than one muscle group is always working to move the weight, which adds more muscles mass. The dumbbell shoulder press for example works the shoulder complex but triceps and the upper back muscles are also benefitting from the movement
• May not seem like much, but the action of carry the weights over to a bench for example can add to the increase in overall strength, opposed to just plotting down and moving weight
• 6 points

Advantages of Machines

• Don't have to worry about wasting time when adding or subtracting weight
• Isolates the targeted muscles in some cases better than free weights
• Breaks up the workout nicely by adding variety to a program
• Some machines hit certain muscle areas that are difficult to reach with free weights, pec deck for inner pectorals for example 
• Injuries may benefit from certain machines over free weights. In the case of a rehabbing torn ACL the leg extension machine is preferred over the barbell squat to prevent atrophy
• 5 points

Winner = Dumbbell Training

While dumbbell training in most cases is the superior method of training, there really is no right way to train. Any new stimulus whether from a machine or dumbbell will increase strength, power and muscle mass. Remember to always break up the workouts with varying reps, sets and rest periods. This will constantly give new stimulus to the muscles. This can be achieved by adding a little machine training into your dumbbell training. Stay strong and healthy.