There's advantages to each. Dumbbells require more effort in stabilizing them, and are excellent for keeping the sides of your body balanced -- whereas a bar allows you to 'cheat' one side a little more.
On the flipside, using a barbell generally allows you to press more weight. Bigger weights = more growth.
They're both fine exercises, and if you really want to grow, switch between them every few weeks to keep your body guessing. What matters more is that you're do compound movements, and hit them as hard as you can. Everything else is sundry.