Forget about the seat tube length and standover. It's the top tube length that matters most. In your case, I'd go with the 49cm (XS), because you have relatively long legs relative to your torso, and the TT length on the 52cm (S) frame is a relatively long 53cm. Also, these bikes have very long head tubes, and if you get the 52cm, you may have to live with a lot of bar rise.