In general, it is much more difficult to stabilize and control barbells and dumbbells and, as such, the work done with these free weights is most conducive to superior muscle growth. That said, machine exercises offer the ability to focus on isolating the targeted muscle group and as such do serve a significant purpose especially when warming up or as a finishing movement. Depending on your goals and your approach to lifting, you may choose to combine each, or emphasize one over the other. If the latter, we're hoping you're talking about free weights.