Here is the part 1 of my evaluation (first look) I suggest you read it before reading this post.
After few days of use I started to notice that some metals have begun to tarnish, here is my further look on tarnish levels of all the metals.
Stainless steel has no tarnish, retained its polished finish and has absolutely no smell.
Copper almost instantaneously picked up fingerprints and gradually became darker. I have also noticed that copper gives out a very strong metallic smell, not sure if this will effect coffee in any way but its possible that it might effect the taste. As you can see from the pictures the metal have noticeably tarnished.
Aluminium has held up really well, absolutely no tarnish and no smell
The brass has started to tarnish, but not as much as copper. This particular metal has actually stained from a drop of coffee, you might be able to see the little stain on the picture. I have tried to clean it but it looks like the metal has absorbed the stain pretty good. This base has a light metallic aroma but not as noticeable as the copper babe.
It will be possible to revive the original shine by cleaning brass and copper in a regular descaler solution, although reg barber recommends to avoid cleaning your tamps with water. I would imagine as soon as copper and brass are exposed to air and constant use they will tarnish again.
If you are going for a long life, polished, shiny looking base than I strongly recommend stainless steel or aluminium.