I doubt that not having a mirror reduces vibration to the extent that it's visible in a negative.
Sharper negs because of better lens design-yes. especially in wide angles.
With RF cameras there is less shutter lag between when you press the release and when the shutter begins to move. In Leica it's in terms of ~10ms VS ~25ms in a SLR. The Contax doesn't have a mirror but does have AF that will incur some lag.
Regarding the ttl viewing, if you're not doing close-up, the minimal parallax error can be discarded as a concern although I've read threads where people are concerned about error with 21mm lenses & auxiliary finders! Oh, Please!
The nicest thing about SLR's is the isolation of the image through the finder. You have the composition surrounded by that nice black frame.