Stick with Nikon (or even Canon) I have a nikon D800E, and there is no end to lens options, plus the viewfinder makes it so much easier to 'read'. I'd also check to see if they make D600/D800 focusing screens, last time I looked there were no quality split screens for my D800E. And manual focusing with the rangefinder arrows is iffy at best. If there were split screens, I'd probably use manual contax lenses on the nikon.
As for the Sony compacts, low light, low aperture and high ISO does not compare to the newest nikon FF bodies (even though sony builds the censors for the Nikons, they are physically larger than the sony mirrorless cameras.)