I'll go with the opinions that carbon frames are better at shock absorption, and stiffer for better power transfer. What that leaves is do you think those factors are worth the extra +$500 you'll pay over and aluminium frame with the same components hanging on it? To me it wouldn't be worth it; I'd rather save the +$500 for extras (pedals, seat, etc). I also personaly like attachment points for fenders and a rack (to carry stuff on long rides - I don't like backpacks) which I've never seen of a carbon frame. I also don't want to be too overly concerned about fastener torque when working on a bike, so the extra margin of error an aluminium frame is a comforting thought (yes, I know you can strip out Al threads, too).
Just my 2 cents worth. FWIW: the only carbon component I have is a fork on a CX bike that only sees road rides on pavement. It seems to work OK, but I really can't discern any difference from a good steel fork.