Re: SS vs Al
It's a tough call between SS and Al.
My guess is, it's a marketing ploy to put higher price on SS model.
No matter how you slice it, I cannot think of any significant difference in the cost of material and manufacturing cost for these two materials in this tiny application.
These parts are probably stamped, perhaps even using the same die. Dies are fairly expensive and I cannot think Apple use two different dies for these materials.
Same cost, but a huge difference in selling price, hence far more profit by hyping SS model.
Now, re material strength, SS is obviously far stronger. Assuming the thickness of SS and Al models is the same, then the thickness of the housing is assumed to be same. If so, there is a possibility that Al case, under certain conditions wherein some excessive pressure is applied inadvertently, it might dent in, or distorted (very unlikely for such a tiny housing with ribs), damaging the internals or permanently deform the watch. In normal wearing, I would suppose the possibility of such accident is close to nil (what is something small is placed underneath the watch, and somone accidentally pressed on it).
But there is a reason why decent (traditional) watch never uses aluminum housing.