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.