Barbells isolate more pecs than dumbbells do, both of them are good for chest though. Dumbbells require more triceps involvement and other arm muscles to stabilize the arm. Think of this, if you are spotting someone clause to failure using barbell press, all you do is putting your hands under his elbows that's it. By doing that you assisted his pecs and delt not his triceps, if you do the same thing with someone using dumbbells, he may let the dumbbells fall over his face due to triceps and elbow muscles fatigue.