What Paul said is true. If weight is your concern then you can do some homework. Get on the various arrow makers' websites and you'll find that there are carbon arrows that weigh as much or more than most aluminum.

Either arrow will do a good job. I think where most guys go wrong is worrying about weight and Kinetic Energy. They crunch all the numbers and go buy some cheap arrow that does not have consistent qualities. Of course, many of them are shooting $700 bows, but inferior arrows, when the arrow is what delivers the broadhead to the intended target. All the bow does is launch it at a given speed.

Whichever you choose just get the best you can afford.