two different things
Stainless is a type of metal or steel. contrary to popular belief, stainless steel does "stain" and requires some care.
Nickel is a plating over a regular metal surface. There are various nickel chemical compounds to get the desired surface - satin, high gloss, industrial, etc.
If you have the two surfaces together side by side it is pretty easy to tell the difference. From some short research the nickel was used because it created a very hard protective barrier for the older corrosive powders and it looked nice as well. Nickel is used on all sorts of appliances, handles and other items where a hard protective finish is needed.
Stainless is all the way through the metal and it can be sanded and scratched. It can also be buffed to remove those small scratches as the metal is all the way through the same.
At least that is what I know.
I just purchased a single six with a couple of minor scratches that I need to start in on.