All things being equal you'll get better penetration using carbon. But all things aren't always equal. The general reason to use carbons is to gain more speed--lighter arrow. BUT, these days there are carbon arrows that weigh as much or more than comparable aluminum. To this end carbon arrows being stiffer, they recover from any kind of paradox quicker and get more of that mass right behind the broadhead, thus carrying more momentum into the hit. In choosing such arrows you'll gain the added momentum and still have an arrow that is less likely to bend or break upon impact with bone, or any obstruction when practicing. And th slightly smaller diameter means a bit less friction also.

Does this help any?