read this from wikipedia...
"In certain markets, such as Taiwan, India, and some European (usually Eastern European) markets, the One S will use the older Snapdragon S3 MSM8260 SoC, with a 1.7 GHz dual-core Scorpion CPU and Adreno 220 GPU. In some markets, both devices are available, notably the UK. The change is speculated to be caused by a shortage of Snapdragon S4 chips. The two versions have slightly different packaging, the S4 is model Z520e/Z520m and states "Processor: Dual Core, 1.5GHz"/"CPu Speed 1.5 GHZ, dual core, whereas the S3 is model Z560e and states "Processor: Dual Core".
So my guess is go witht he z520e if you want one that will work in the US 100%