According to a game warden who works on the Gulf Coast, nickel is more corrosion-resistant in salt air than stainless. However, holster leather and some storage materials (like styrofoam) tarnish nickel more easily than stainless, in my experience. The tarnish can be removed with a non-abrasive polish, but only on bright nickel surfaces, not on satin nickel (most any polish will alter the reflectivity of satin nickel, which is best left alone).
Finishing in nickel costs more to make than most stainless finishes, which I assume is why firearms manufacturers have largely dropped nickel for stainless. But I think nickel is better looking than stainless. To my eye, nickel has a slightly golden undertone while stainless has a relatively blue-gray look.
IMHO the best nickel finishes used to come from Colt's. S&W did a respectable job of finishing in nickel as well. Back in the 80s BHPs were available in a brushed-looking nickel that was attractive, but somehow didn't quite measure up to the bright nickel or satin nickel of Colt's O-frames.
Jim K. offers some good advice on the care of nickel. I keep the use of solvents to a minimum on my nickel guns, and only lube with Hoppe's Gun Oil.
FWIW, I've seen one Colt (a Combat Commander from around 1980) with nickel peeling in the finger grooves of the safety and the slide. I don't know if Colt's or the owner messed up. Also, I remember a brand-new S&W Model 13 with a severe peeling problem on the barrel--no doubt the factory's fault and very unusual. I owned a nickel BHP and was not happy when the plating started to peel off the hammer right away. However, Browning's service department took care of the problem.
My $0.02.
[This message has been edited by jimmy (edited November 10, 1999).]