In the end of all things it will just be what you end up being accustomed to, and what you need out of the camera. If you have been shooting with Nikon for years, the controls will be very familiar when you start going to newer cameras.
What do you feel you will end up shooting? Weddings? Night shots where there is 0 available light? Sports?
If you are not going to be shooting sports then 9+ FPS will not be needed, so you can scratch a D3X, D4(s), 1Dx
If you are looking at shooting weddings then speed is not what you need as you will take your time framing those important shots and trying to make everyone comfortable as you pose them. D600(610), D800, D700, 5DmkII, 6D, etc will work with this.
If you plan on shooting concerts where you will always end up finding yourself losing light quickly then you want to find yourself with higher range ISO, but it would put you back into cameras that are made for low light and fast frame shooting.
In the end, if you already have Nikon lenses, jumping ship to get all new equipment might be counter productive, but it'll be your decision in the end.