I was also looking for One X, since coming from a Galaxy Nexus it felt just as the right screen size. After playing 2 days in a row appr. 30 minutes each I walked away with One S for the following reasons.
1. Build quality is absolutely gorgeous - One S has the best look & feel of any mobile in the market today. It`s also the first ultra lightweigt phone, that does not feel cheap in my hands. The One X was a little bit too wide, so one handed operation was to tough - Galaxy Nexus was still in my comfort zone, One X just missed it. Also the build quality does not match One S, even if it`s still better then most competing devices out there.
2. Speed - the One S is the snappiest Android device I ever tested. Everything is incredibly smooth, One X was even slower then my current Galaxy Nexus (which is certainly not really slow...).
3. Display - even if font details can not compete with X or my GN I prefer the better blacks of the S compared to X and the better whites compared to my GN. Also this Amoled is not blueish or greenish, like S2 or GN.
4. Sense 4.0 has finally grown up - with my old DHD I really had to throw Themes at it to use it without throwing up. Now Sense 4.0 has a fairly sopisticated and professional feel and the glossy, childish look of older versions completely disappeared.
Overall for me the One S is a real understatement phone, which would be the true flagship phone for HTC in a different universe, where size & more cores are not the only measurement for being treated as a flagship. Just compare One S to Cinderella - their mother (HTC) could try to show their sisters (One X) as the real deal to the prince (me) as long as she liked, the prince ultimately prefers the beautiful step daughter (One S)