Can't speak for the Ion, and I'm gonna sound totally pretentious but it's the smaller details - there's something really slick and minimal about the Xperia S. No rounded corners, that bar across the bottom which is transparent and lights up really softly, the matte finish and how the screen seems to disappear into the casing when it's off. I fucking love this phone. Switched from my iphone 4 (which I loved - the bf is getting that one) but the geek in me is loving Android. It's far less clunky than I've been led to believe - it's not even ICS yet either
At first glance you think "like the iphone4" but the more you look , the fine details emerge and it really looks slick. But i like round corners . Galaxy Nexus ist still one of the sexiest for me. One S is a bit bland but also nice.