I have an SS (purchased when AW launched), plus an Aluminium Space Grey Nike Watch (purchased last year). These days, I typically work the SS four days a week (at work, with suit) & the Aluminium three days a week (with casual wear). Despite being ultra careful, I have a slight scratch on the face of the Aluminium watch, but the SS has no scratches. In terms of weight, I do not notice any difference between the watches - the SS does feel more substantial, but not 'heavier'.
One rather obscure observation - when wearing the SS watch, I often struggle to get Apple Pay to work with the turnstiles on the London Underground...it takes numerous attempts before the turnstiles will open....but when I wear the Aluminium watch, the turnstiles open as soon as I 'flash' my Watch at the contactless sensor!