Personally, I think ss retains the "zing" a bit longer, however, they don't settle in quite as nicely as nickel. I don't like the sound of well worn ss..... well worn nickel sounds great.
That is subject to a great number of variables, such as brand, gauge, core wire, etc. I have noticed that stainless lasts probably longer in the sets I have tried, but I am not enamoured of their sound and feel as a general rule. Nickels seem to fit what I do better, so I stick with them.
Which lasts longer..... 
