I have found that the stainless steel used on iPhones and Apple watch are extremely scratch resistant. I'm currently using a series 5 apple watch and have an iPhone XS Max, both gold. They come with me everywhere and both look like the day I bought them. The Watch is particularly impressive considering everything I use it for.
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I semi-disagree. The 316L stainless that is used on the Apple Watch, is the same stainless steel used on the iPhone bands as mentioned, however; I definitely wouldn’t go as far as being extremely scratch resistant. As matter of fact, it’s the complete opposite, it’s a very soft/dense metal, scratches very easily, it can just as easily be removed with a polishing cloth/paste. Unless of course, you opted for the space gray stainless, which has the PVD [Physical Vapor Deposition] coating on the iPhone bands, or the DLC [Diamond Like Carbon] coating which is applicable to the space black Apple Watch, Those two specific coatings will make a difference in terms of scratch resistance.
If you’re using a case, that would help against the protection, but generally by experience, (especially if you visit the Apple Watch forum with those who have the stainless model), the stainless scratches very easily.
Even the Gold stainless bands with the PVD coating, unimpressively still Was not very scratch resistant either.