Generally speaking, carbon arrows fly faster. They are smaller diameter, lighter and less wind resistance. This reisistance is what allows the carbons to flyflatter. Having said that, one of my bows shoots aluminums 292 fps. Oh, it's also a 7 year old bow.

You can get lighter aluminum arrows as well. There are quite a few out there. I personally like carbons better because they take more abuse than aluminums. You can bump and bang them together on the practice range and they are less effected. Not that they don't get hurt, though.