Get the One S if you want to go stock. The S4 is much better than the S3 that's in the Galaxy S3. I know because my Galaxy Note has a S3 and my old One X has a S4. Both clocked the same and had the same amount of ram, but the One X was so much smoother which leads me to believe that the S4 is far more optimized even at the same clock speed.
however, the S2 has a great modding community and if you want to flash roms, it's hard to go against Samsung. So much easier to do that kind of stuff. Now my next question is why not go with a Galaxy Nexus? That would be better than anything since it ships with stock Jellybean right now.