All this SS discussion made me curious and I returned my 15-day-old aluminum Sport today (thank you, Apple) because a local store had a SS/sport band (didn't want gold or SB, and would never wear the Milanese loop, no matter now nice).
First impressions:
It's probably a trick of the eye, but the bezel on the bright SS Watch almost disappears. The silver aluminum had really small bezels. The bright SS seems to have none, which is cool.
Whatever the actual numbers are, the SS feels much heavier than the aluminum (both on a Nike+ band). I don't think it's a big deal, but - to me - it is very noticeable.
I really love the look of the bright SS. It seemed an extravagant premium to pay for a tech device, but even if that's true (and I still think it is), it certainly is great looking.