Ah! A new thread that brings up ye ole' Al vs Cu debate!
It may not be obvious with your design, but copper is superior. That aside, please continue using Alu, because you can still make a decent water block with it.
Copper has a better thermal conductivity (do you really want the numbers? I didn't think so! ) but also has a (relatively) high thermal capacity.
I would bet hard money that your waterblock baseplate is about 1/4" thick. If you could make the same comparison, but with a baseplate that's about half as thick as what you have now, you would see a significant difference.
Do you want more info?
BTW, don't post your numbers here! We're on a testing "kick" right now, and we're all about accuracy. That CPU temp has an accuracy of +/- 10 deg C. Report temps when you're at +/- 0.1 deg C, whichever way you get there...
