Slow reps are BY FAR better than fast reps. There are many reasons. Here are a mere sum of them. When you lift slowly you have good balance and help take away the risks of serious injury. It works your muscles harder because the stress of the weight is held for a longer period of time. Slow lifting can also strengthen your tendons and ligaments as well as your bones. The only type of slow lifting I would avoid thhough is super slow lifting. This kind of lifting is balanced, but puts too much stress on bodyparts not even associated with the exercise your doing. You could injure yourself badly. I hope this helps you and good luck with your goals.