black stainless has the DLC coating that makes it very, very hard to scratch. many people report that after a year or two the black stainless body is still flawless or has less than 1 or 2 microfine scratches that can only been seen in light and have no depth and are nearly invisible. if for some reason it did truly become gouged there isn't a way to polish it however, but i honestly have never seen anyone report anything like that it is so strong.
the regular stainless steel will get scuffs and marks before the end of your first DAY however within a month or so it forms a nice patina that adds to the character AND looks normal and nice for stainless in my opinion. that said a cheap shining cloth for stainless can every few months be used to remove 90-95% of the marks and return the phone to nearly new condition although of course almost immediately it will start getting new ones.
both stainless but very, very different properties. both are beautiful and a huge, huge upgrade from aluminum.
PS: I agree with what everyone says about sapphire, its miracle stuff. i abused the heck out of my watches with sapphire (Apple Watch series 0, garmin fenix) and both look like the day I bought them after smashing the screens into metal corners and brick. I am moving back to AW for 2018 (I'm crazy, I bought myself a SBSS AW3 that is sitting here but have decided to wait until 1/1/18 to open it) and you bet I wanted sapphire again.