Stainless steel frets are NOT nickel frets, so is their a difference - of course! However minimal or minute it would be, there are dozens of differences (if you break it down scientifically). And, they can be "compared" in each individual category.
I actually "love" both, although if I had to choose just one, I know which one I would have to pick. I hate the "wear" on nickel, and I enjoy the buttery smoothness of ss. There may be a slight "ping" of treble with the ss, but one thing
you can't do on ss is "burn" a note. (play a note and wear the fret up and down for until it burns). Of course this will really mow down the nickel frets in no time.