I have the MX-1 and the K-S2, and I agree that the MX-1 would be MUCH more appropriate for stealthy street shooting.
The K-S2 is too big, the screen articulates any way you want, but it has to fold OUT to the left first, making it much larger than it already is (as mentioned above).
The MX-1 is small, the screen tilts up directly behind the camera, so the size stays as compact as possible. The shooting/focusing modes are perfectly versatile for street shots, AND, having worked with the RAW files, don't you worry about that small sensor. The images are fantastic. Plus, the smaller sensor is going to give you greater DoF, which I think is probably preferable for candid street shots.
One of my favorite low-light shots with the MX-1:
For your purpose, and owning both cameras myself: Get the MX-1... But if you want the K-S2, just get it.