Nickel is the material the base (the part that makes contact with the processor) is made out of. Typically, if it says "Nickel" that means it Nickel-plated Copper. It is also possible to get a normal copper base and even a Gold-Platted copper base in some instances. The differences between cooling performance are negligible enough that the difference ultimately comes down to aesthetic preferences.
The top of those EK waterblocks are made out of either Acetal or Plexi (other brands might use different materials). Acetal is basically a metal top and Plexi is plastic. Acetal tops are more rigid and generally less prone to deformation (which can cause leaks), especially on large GPU waterblocks. However, this deformation isn't that common, especially from a reputable brand like EK. The only advantage Plexi has over Acetal is that it's usually transparent so if you're using colored coolants or dyes, you will see the fluid flowing through the waterblock. Just like with the base materials I mentioned above, the difference between top materials is largely aesthetic. Of those, it doesn't matter which one you go with so long as you like the way it looks. Nickel-Platted bases are popular because most people prefer the way it looks to copper, especially when used with a transparent Plexi top. If you use an opaque Acetal top, the base won't show nearly as much.
There is also the factor of mixing metals in your loop. Anytime you mix different types of metals, there will be an ion exchange that could could potentially corrode your waterblocks and even cause the coolant to become electronically charged. When it comes to mixing Copper/Nickel/Bronze, the effect of this is negligible. The only time this is really important is when you're using Aluminum components. Mixing Copper/Nickel/Bronze with Aluminum is a bad idea. If you want to be absolutely safe, you could get a Copper radiator, a copper waterblock base with either a copper top or a plexi top and then use plastic everywhere else. Frankly, that type of safety measure is unnecessary because it is safe to mix copper with nickel. I only bring it up because it is one potential reason why someone might choose one material over another. I personally have Copper, Nickel, Bronze, and Silver mixed in my loop.
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