I have both copper and brass. Copper is a high maintenance material. Needs to be cleaned every couple days to keep threading and parts clear of tarnish. Brass is more user friendly.
When you are looking at these materials be sure to inquire exactly the material of the mod. For Copper you want C10100. This is oxygen free copper and is the purest one can find. Best conductivity of all copper as well. Some makers are using lesser quality copper that will turn green from tarnish. 101 copper goes black. After heavy use for a couple days of no cleaning, the tarnish will come off on your hands. My thumb crotch will get black after a few hours.
For brass you want 464 Naval Brass. Good conductivity and tarnish resistance. With a good mod, it can perform negligibly less than copper. With either material Cape Cod Cloths and microfiber will be a good purchase.
Lastly, with copper, be prepared to either up the resistance of your build, or lower the nic levels. I was at 18mg when I started and have now moved to 6mg. Not because I am looking to quit (luv the nic), but because the hits are stronger due to lower resistance of the mods.