Nickel plate is just for looks, doesn't make it perform any difference.
The supremacy is supposed to be less restrictive while being better at cooling than the supreme. How much better would depend on your loop and components, but to be honest watercooling is an expensive plunge and cheaping out on a block is not the way to go about it, so if iw ere you i would grab one of the blocks int he above post. The supremacy also comes with the original way EK mounted CPU blocks, while the supreme no longer does and its mounting holes on the block is built into the plexi top (which imo looks a little cheap).
I think out of the three of them, the supremacy is the only one which comes with a mounting system which can be changed to suit any socket, while the other two are bought either with AMD or intel compatibility (you may not be looking for an upgrade now, but its worth future proofing). Also worth noting that block top spares can be ordered from EK for fairly cheap (something like a tenner) if you decide you want a new look or want to replace a cracked block.