I owned the One S for a week and am now on the SGS3 so I figured I'd throw in my opinions. I came from a SGS2 (sprint epic 4g touch) but really loved the 4.3" form factor of the One S so decided to go with it. Love the hardware design and materials on the One S, definitely better than the galaxy. I've hated Sense in the past, but this new version is very nice - I prefer it to Touchwiz. Also, and I'm in the minority here, but I LOVE the dedicated task-switcher button on the One S. I don't necessarily love that they did a cover-flow style UI, but even despite that it's just so easy and quick to switch apps that I found myself not missing a "menu" button at all. I also prefer the capacitive home button on the One S compared to the clunky and stiff physical home button on the SGS3.
But the thing that ultimately killed the One S for me was the pentile screen. After 24 hours with the phone I knew I'd end up returning it, it was just so grainy. I know screen quality is subjective to some degree, but I'm convinced that the people who claim the screen looks fine either need to have their eyes checked, or have come from a much older phone. Put it this way: the screen was a noticeable downgrade from the SGS2. I don't mind it not being HD resolution, but I just couldn't handle taking a step backwards on something as important as the screen. (and especially not for that price)
Bottom line: had it been a non-pentile screen, I might not have switched to the SGS3. (but I do really like the SGS3) With the rumors that the One X is coming to TMobile, I realize I maybe should have waited before pulling the trigger at all.