Alluminum arrows are heavier than carbon arrows. Among aluminum arrows there are usually three or four different weights that will have the same spine(stiffness) this is usually achieved by thinner walls and larger diameter in the shaft. Carbons vary on weight as well so you can choose what traits you want in a carbon shaft.
I am in the heavier arrow camp. I find that aluminums are working just fine. I can't say that carbons have some inherant down side I just get good enough results from the aluminums.
If you are just starting out, do yourself a favor. Don't chase speed, chase forgiveness in your setup. At every opportunity to add an accessory ask which is the more bulletproof option. Better yet, do I need this hanging off my bow?