The M6 shutter is quieter, as you would expect a cloth focal-plane shutter to be. The Zeiss Ikon is well damped and is noticeably quieter than a Bessa-L, R or R2.
If your primary concern is quiet, then a leaf-shutter camera will be best. My reigning "king of quiet" is a Retina IIIS.
I shoot outside quite a bit with the camera, and no one gives me a second glance when the shutter is released.
In general, the whole thing about the decible level of the shutter receives entirely too much attention. For most of us, there are only a handful of circumstances when you really need a quiet camera. For most shooting, it's not a big deal -- even if you're using a big mirror-slapping SL66.