I would like to add there is a slight difference aside from aesthetics, Each metal has an electrode potential ranging from cathodic (resistant to corrosion) to anodic. When metals are in contact electrically there is a potential for galvanic corrosion, the higher the difference in electrode potential the more likely galvanic corrosion is going to occur.
Example looking at that chart Zinc will corrode extremely fast if in electrical contact with something highly cathodic like gold as the metallic ions from the zinc are transferred to the gold. The lower the difference in electode potential between the metals the slower corrosion will occur, now if we look at something like copper (-0.4) and nickel-silver (-0.3) which is most commonly used nickel for electroplating there is a difference in electrode potential, it's not huge but enough to cause some corrosion over time. In a water-loop the water blocks are electrically in contact with fittings, radiators and other blocks via the water in the loop. If you look at silver it is more cathodic at -0.2 and this is why a lot of people who use silver kill coils tend to experience faster corrosion.
If you are going to go for copper I would try and suggest you get everything possible in copper, likewise with nickel try and get all the same metal in the loop.
The difference in nickel and copper is not going to destroy your loop however it will cause quite a bit of discoloration in the copper blocks and require more regular flushing/cleaning to maintain. The gunk that people clean out of their blocks is often a combination of dye deposits from the coolant and some oxidization from galvanic corrosion, in the picture below its mostly oxidization and some dye depositing in the centre.
Some companies leave the copper on their blocks raw and others put a protective lacquer on their blocks. EK does not lacquer thier blocks but Heatkiller and Aqua-computer do lacquer their blocks. The raw ones (EK) are completely fine to clean with lemon juice, tomato sauce, acid or whatever mildly acidic solution takes your fancy however I would urge you not to clean lacquered blocks with acidic solutions as it will completely strip the lacquer of the block (although you can get them re-lacquered).