Depends what are you using the camera for. If for hobby, the depend if you have the $$ and willing to spend.
D600 is Full Frame Sensor, obviously there will be a lot of difference overall. Again, depends what are you using the camera for? what do you shoot? do you need it? Got $$? Willing to spend?
Upgrading to FX you might want to get an FX lens to fully utilize the focal length and MP (FX lens means there's more $$ involved aside from the body itself); of course, you can still use the DX mode and make do, all depends on you.

One more thing, put your eye to the viewfinder on a FX body, you will see something different already.

Shooting VIVID the color will over saturate, also depending on the exposure and lighting, etc.
Safer to use standard or portrait for human, and manually adjust vibrance/saturation/etc later. If you shoot with high saturation as default, not much room for recovery later as the file is already processed. It's like if you overcooked a meat, then you cannot undo later.