It also depends how picky you are. Like you, I've had the aluminum version for years, and it's held up amazingly well even with bumps along the way. That being said, I have had a few instances where I did manage to scratch up the screen. This is in completely understandable situations like hard scraping against very coarse rock/brick/etc. So my screen does have noticable (if you look closely) scratches along the display. I've never been too anal retentive with regards to keeping pristine devices, so I'm (mostly) fine with it. But it does make me consider the SS model going forward. I would definitely care more about the saphire front and back (I have lots of "microabrasions" along the HR monitor, I'm guessing from my arm hair) than the SS material.
But that being said, buying the SS for me would mean I'm planning to go 4-5 years with it... but upgrading every 2-3 years (aluminum) at the same cost seems like a better option...