2 bows both are shooting the same fps. i have chrono' d them. one is shooting alum arrows w/ 125 grain muzzy' s, the other is shooting carbon w/ 100 grain muzzy' s. i have a " the block" target. the alum 125 grain penetrates so far only about 1/2 of the fletching is sticking out at 20 yards and at 30 yards they penetrate almost to the fletching. the carbon 100grain leave the fletching and about 1/2 inch of the shaft sticking out of the target at 20 yards and 3-4 inches of the shaft at 30 yards. am i missing something or is it just the stored energy is higher in the heavier arrow than the light one. considering they are w/in 3-4 fps of each other would it be better (more effective/humane) to shoot the heavier arrows or will they both do the job effectively?

thanks