Carbons for me (shot Aluminum for about 15 years then went to carbons...will NEVER go back)
Once you find the right spine, the carbons are tougher, and you can get carbons with VERY tight tolerances (sp?)
ALL else being equal (diameter/weight/speed/spine/head/length...anything else you want to add...) a carbon will penetrate BETTER than aluminum as they don' t oscillate as much (or as long...basically have built in damping)...this is also a BIG aid to accuracy when shooting broadheads at high speeds.
Not everyone can shoot 5 spot targets all the time (at hunting camp we only have 2 targets for 4 guys....you pick 1 spot to shoot) when shooting more than 2-3 arrows into a group, your GONNA hit some arrows together. I got sick of buying new aluminums all the time due to dents in the sides from other arrows...now I don' t have to.
MOST of the nocks agains' t carbon are NOT agains' t carbon proper...but agains' t light weight (broadhead tuning, KE or pass throughs, quietness, easy on limbs etc).
Can' t tune a noisy bow with carbons? Add some wieght.