2) it weighs more! This is good because if you ever overclock so much you fry the processor, you can remove the heatsink, and throw the heavy bugger at the nearest person, and get a satisfying crunching noise. GREAT for stress relief.
the difference between the two is so marginal that you probably won't tell a difference. check out some reviews on the Cu and AlCu to see how they compare, it is usually within 1 degree.
I was wondering. If the AlCu model has only a band of copper in the middle, wouldn't that mean that most of the P4 IHS is getting aluminum contact only? I mainly use the AlCu for AMD builds, and the copper band in the heatsink sits right over the core of the AMD chips (AthlonXP).