While I certainly favor a Nikon kit and would encourage you to follow your instincts, I'm not exactly clear on what problem you have with your Sony that you expect would be solved by the D600 or D800.
I wouldn't worry about the pixel pitch creating more or less demand on your shooting. Ultimately, if you shoot the same scene with D700, D600, or D800, and then output to the same size, you will have magnified everything, including flaws in equipment and technique, exactly the same amount. The "more pixels are more demanding" myth comes from people zooming in to 100% view on their computer screen, at which point you are magnifying the small pixel image considerably more than the larger pixel images. But in the end, there's no real use in viewing an image that way. There is, however, certainly a valid argument that you don't really need 36MP unless you plan to print really big. (And some of us do.)
You may want to hold the cameras in your hand and see which fits better. You won't get your hands on a D600 quite yet but body size the D7000 should be roughly comparable (although the notable difference will be the larger D600 viewfinder to accommodate the larger FX prism).