They're pretty much on par. One S has a terrible wifi antenna. Screens are almost the same. One S is brighter. More storage with the S2. Cameras are on par. Thought I like the front camera on the One S better. S2 is bigger if that matters. Me personally I would wait for the S3. However if I had to choose between the One S or the S2... S2 all the way. Hope this helps you.
I got my SII from Costco, where they had both it and the One S on display -- both working phones. I was torn, too, until I saw the SII's display. The One may have better resolution, but the display makes the SII the clear choice (I can see the pixel grid on the One -- mainly because of the display layout (PenTile).
I had the same issue when my BlackBerry was long overdue for retirement. I looked and played around with both phones. With ICS, the S2 doesn't feel dated despite it being out for as long as it has been.
Storage was a big issue with me as well as the display quality. While the S2 may have a lower resolution screen compared with the HTC One S, it simply looked far better and photos were amazing.
I'm happy with my choice, dropped in a 32gb Class 10 MicroSD card and rarely found a reason to carry my point & shoot camera any longer.