I believe stainless is better as far as corrosion resistance. Nickel is just a plating. That being said any barrel, including plain old blued barrels, will be fine with proper maintenance.

I can't comment on the black blotches on your nickel finish, but the kodiak is a nice gun. I looked at one last summer when I started shooting ML'ers, but went a different route. Then I got into sidelocks, but that is a different story.

Good luck with the Kodiak.