As has been said, nickel is harder to maintain a good sight picture in varying light conditions. The Single Action Army doesn't provide good sights at best, and shining them up doesn't help.
But if you're not concerned about precision shooting and like the looks of nickel, by all means go with nickel. Both case hardened/blue and nickel will show age in time and develop "character" that a stainless gun can never hope to achieve.
Bob Wright