I'm gonna get a water block for my 4870 X2 card very soon.

I think I've settled on getting one of the three EK water blocks.

Unfortunately the store I'd like to purchase from, PerformancePC.com doesn't have the nickel water block in stock. So I could settle for the copper one for $165.

Or I could go to Frozen CPU. All their blocks cost about $20 more, but they have the nickel one in stock for $184.

I'm not really sure what the difference would be between the copper and nickel. Is it worth the extra $20 or more?

If I just go with the cheaper copper ones at PerformancePC.com, are there any pros or cons with the see through acrylic cover versus the black cover on the water block?