Taken from the Apple site:
"Surgical‑grade stainless steel.
The stainless steel band that wraps around and reinforces iPhone X is a special Apple-designed alloy that’s both durable and more pure, and it polishes beautifully. For the space grey finish, we use a process called physical vapour deposition to precisely match the colour of the stainless steel band to that of the glass."
Now I like my devices/jewellery in black or the darkest option so I originally desired the Space Grey.
Then I found out the colour on the stainless steel was a coating, so fearing scratches that can't be polished out like with plain stainless steel, I decided on the off white(?) iPhone X.
Then I researched PVD coating and discovered that it is more hard wearing and scratch resistant than bare SS.
So now I'm still a bit undecided. I'm confident any minor abrasions/scratches on the plain SS will polish out no probs with a Cape Cod cloth or similar.
But I prefer the look of the Space Grey and the PVD coated sides may be harder wearing but how much so is unknown.
What would have been ideal would have been the same DLC (diamond like carbon) coating the Space Black Apple Watch has. I've had mine since launch (series 0) and there is not one blemish on it. But alas, no.