I've always liked training with free weights and especially dumbbells. Dumbbells give you freedom of movement and also work your core with certain exercises as well. You can also alternate between arms for a variation of the exercise which is not possible with machines. Once you've mastered the balance part for exercises like dumbbell presses and flyes you'll be able to better focus on the muscle group you're working due to the isolation nature of the movement. This in turn will lead to greater gains in strength and mass.
Here are some excellent dumbbell exercises for arms with PowerBlock adjustable dumbbells.
View video for PowerBlock Arm Exercises.
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