I agree that the resolution or photosite per area on the sensor is higher on the D300. I don't agree with the statement "I feel this is an important thing to consider for those DX users thinking of upgrading to a D600."
True resolution has more to do with pixel count on a subject. For example if you fill a frame with a persons face, the D600 will have far more pixels on that persons face than the D600 24MP vs 12 MP.
Also by your logic my i-phone has better resolution than a D600 as many tiny photosites are crammed onto a tiny sensor. No one would consider this and decide not to get a DX or FX DSLR due to the higher resolution sensor.
I feel taking lens behavior into consideration and ensuring you have FX lenses is a far more important thing to consider when contemplating DX to FX switch.
"Cameras and lenses are simply tools to place our unique vision on film. Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colors and your images will stir the soul." Jack Dykinga