AlCu is lighter and won't bend your motherboard, but it cools almost as well (within 1 degree in some tests).
Unless you care about the absolutely highest possible overclock (way above "normal" overclock), get the alcu.
My brother has the alcu on a P4 3.2 GHz and it runs 10-20 degrees cooler than the retial HSF even with the fan speed dialed down.