Nickel is a surface plating that if done well - effectively provides protection against the elements.
During the Frontier Era - it was sort of the 'stainless' finish of today.
Being a surface plate - it has a 'softness' to the exterior finish and will mar and show scratches.
Stainless Steel on the other hand, is just that - stainless throughout the entire firearm and as such - gives a harder, more scratch-resistant finish.
Under the right conditions, 'stainless' will stain, though - so you will have to maintain the piece - just not as often as you will a blued piece.
The 'traditional' finish on period firearms was Blue and Casehardening.
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