The part contacting the GPU is nickel-plated copper on both, if I'm not mistaken.
The difference is purely visual:
- The nickel + acetal blocks are nickel-and-black, so they look more stealth
- The nickel blocks are nickel-and-acrylic, so you can see through them and see the liquid go through the block
Personally, I went with acetal on my last build, but I'm going see-through this time.