Again, FR3D1, waterblocks for what?! We need more input about your hardware...
Generally speaking, copper and nickel are usually the contact surfaces of waterblocks - so you won't even see to much of them after installation. The plexi and acetal is the part of the block which will face you (except VGAs - there you will see only the edge). So, you want transparent or not? Plexi is usually transparent, while acetal has some color (usually black or white)...
Getting back on copper vs nickel on functional side, copper has a higher heat dissipation ability - but in the case of the waterblocks that nickel plating of a fraction of the millimeters won't really matter.
What really matters at waterblocks is the interior design - how restrictive they are. Usually less restriction they have, better will be the coolant flow. But even if you have high restriction, you can use stronger more pumps...
One thing you have to keep in mind: don't mix copper with aluminum! I mean, if you choose a copper block, you have to et a copper radiator too - otherwise you will end up with electrolitic degradation of your entire loop.
Bottom line is: Get whichever you like better! When you post some hardware specs, we can discuss which brand/model is better for you. And keep in mind the not-to mix copper with aluminum rule!