Yes, you can just buff the scratches on the stainless steel but by doing this a lot, you can actually buff away the coating and will eventually see the actual steel under there if you're not careful.
There's no coating. Stainless steel becomes stainless steel by the addition of Chromium and Nickel into the grain structure of the metal. The chromium, when exposed to oxygen, forms a passivation layer on the surface, preventing pitting due to Cl- or other salty ions in the air. You can buff all day and still have a passivation layer.
I guarantee you that your aluminum will ding if you accidentally knick it on something (which is inevitable with a watch). My Tissot with hardened steel bezel has a knick in it because I smacked my wrist into a car door once - had it been aluminum, it would've been totally gashed in. The sapphire crystal though, is absolutely flawless.