But why would they have bothered moving from 6 to 7 series aluminum for the 6s, and why would they invest 200 million in Corning to achieve stronger glass?
I don't know what you mean with this aluminum thing, all I can say is that my 7 and 7+ were so slippery that without a dbrand skin, they had a tendency to slip out of my hands, even when I just sat on my couch without moving.
At least the diamond black 7 was so prone to scratch that it was almost impossible to not scratch it in just the first few minutes of (regular) use.
Afaik the 'hardest gorilla glass ever' was used to prevent scratches on the surfaces or backs of the 8 or X, but didn't prevent the X from getting awarded the 'most fragile iPhone ever' (sorry I can't find that article right now), as it didn't survive most drop tests (Youtube, review blogs etc.), and the glass broke quite easily (compared to other materials). Scratch resistance apparently does not mean break resistance. Even though the X's surface and back might be scratch resistant, its stainless steel sides don't seem to be (as reported here by several users-and I think this is what this thread is all about?).
Finally, now that my X has arrived, I have to admit that it seems to be less slippery than the 7. It feels like oil & moisture stick better on glass than on aluminum. So less slippery, but in exchange less scratch resistant sides, and potentially breaking more easily.
However. It's a beauty. Maybe that's the price one has to pay.